All About Scopes - V

In our last few posts, we studied telescopic sights from the outside and in our previous post, we studied one part that is inside the scope, namely, the reticle. We also talked about terms like First Focal Plane (FFP) and Second Focal Plane (SFP) in our previous post. In today's post, we will look into the parts of a telescopic sight in more detail and find out what FFP and SFP really mean.

Before we dive into a telescopic sight, let us first study lenses, in particular, a type of lens called the biconvex lens (also sometimes called a converging lens). A biconvex lens is thicker in the middle and thinner at the edges. It is typically made of a transparent material, such as glass. Since glass has a greater density than air, light bends as it passes through it (you can observe the same effect if you look into a pool of water, as objects at the bottom of the pool appear to be at a different location than where they really are, due to the light rays bending as they enter it.) In a lens of this type, when light from a great distance passes through it, the light is concentrated to a spot on the other side of the lens, called the focal point, as shown in the diagram below:

Image licensed under the GNU Free Documentation License Version 1.2

The distance between the center of the lens and the focal point is called the focal length of the lens (marked as 'f' in the figure above). Before you start yawning after reading this material, you may actually be familiar with the concept of focal length, but not realize it. As children, many of us have used a magnifying glass to burn holes into a sheet of paper (admit it, you did it too). The trick is to move the lens up and down to adjust the focus, until a tiny concentrated image of the sun appears on the sheet of paper. When the paper starts to burn, the distance between the lens and the sheet of paper is the focal length of the lens.

Now let us consider an object as viewed through a lens, as shown in the image below:

Image licensed under the GNU Free Documentation License Version 1.2

When the object is at a distance S1, at a great distance from the lens, an image can be projected onto a screen at a distance S2 on the other side of the lens (where S2 is much smaller than S1). We will not study the cases where the object is located at a closer distance to the lens (e,g. if it is closer than the focal length or twice the focal length), as these are irrelevant to our object of study.

The main thing to notice in the above diagram is that the image is upside down because of the way that the lens bends the light reflecting from the object. As you can imagine, this is not very useful for telescopic sights because people aren't used to seeing things upside-down. We will study how this is rectified later.

Now that we've studied how light passes through a single lens, let us study how light passes through two lenses (i.e.) a simple refracting telescope similar to one used by Galileo in the 17th century,

Click on the image to enlarge

The above image shows how light passes through two lenses. The lens on the left is the larger objective lens, which has a longer focal length, and the lens on the right is the eyepiece lens (otherwise called the ocular lens). We encountered these terms a few posts ago, when we studied the external parts of a scope.

As you can see from the images above, the two lenses collect more light than a human eye can by itself, and give the user a brighter magnified image. Note that because of the way that the light bends, the image that the user sees is upside-down. The fact that the image appears upside-down doesn't matter when viewing symmetrical objects such as stars, the planets, the moon etc. However, it does definitely matter when viewing targets on the earth, as users prefer to see their targets aligned in the proper direction.

Therefore, scopes work around this issue by having another set of lenses in between the objective lens and the ocular lens. These intermediate lenses also flip the image, therefore when it comes out of the ocular lens, the double inversion causes the image to be turned back to the correct direction, the way that most users like to see it. The image below shows how this works:

Internals of a telescopic sight. Click on the image to enlarge.

Light passes through the objective lens and the image is inverted, as we have already seen a few paragraphs above. It then passes through another set of lenses, labelled as the picture reversal assembly (also called "erector lenses") in the image above. These lenses serve to invert the image again, which means by the time it passes through the ocular lens, the image is flipped back to the correct direction. In variable power scopes, there is a mechanism to allow the erector lenses to be moved back and forth, which changes the magnification power of the scope.

Also note that the first focal plane (FFP) reticle is located at the focal length of the objective lens. There is also a second focal plane (SFP), which is located at the focal length of the erector lenses that comprise the picture reversal assembly. The reticle can be placed at either the FFP point or the SFP point. We studied the implications of placing the reticle at FFP vs. SFP a couple of posts ago and also in our last post.

We will study more about the internal parts of a scope in the next post.

All About Scopes - IV

In our last post, we studied some details about the reticle inside a scope. In today's post, we will look into some more uses of reticles, namely rangefinding. We had actually dealt with this topic some time ago, when we talked about telescopic sights.

The key to rangefinding in most scopes is to compare an object of known height or width against a series of markings on the reticle, to determine how far away it is. We will see how this works with a few examples.

Reticle from a Russian PSO-1 Scope, as used by the SVD rifle. Click on the image to enlarge. Public domain image.

The above image shows the reticle from a PSO-1 scope, which was originally designed for use by the Soviet military Dragunov SVD sniper rifle. When this was originally introduced in 1964, it was the most advanced mass-produced scope available. This scope is a fixed power scope with 4x magnification. It has several markings on it. The top chevron (^) mark is used as the main aiming mark. The horizontal marks (10...10) are used for adjusting for windage and also allow to lead the target, in case it happens to be moving. The horizontal marks can also be used for rangefinding, if the width of the target is known beforehand. Each marking on the horizontal 10..10 marking is spaced at one milliradian interval, therefore the calculation for finding distance can be determined by the following formula:

D = S / mils * 1000

where
D = distance to target in meters
S = Known height or width of target in meters
mils = Number of markings wide that it appears when viewed through the scope

Say the user is viewing a Land Rover vehicle, which is known to measure about 4 meters long (i.e. 13.12 feet long). Let's say that when it is viewed through the scope, it measures up to 8 markings long. Then, we put S = 4, mils = 8 in the above formula and calculate D = 4/8 * 1000, which works out to 500 meters. Therefore, the Land Rover is approximately 500 meters away from the scope.

Also notice the curved line on the lower left quadrant with the numbers (10..2). This can also be used to measure distance to the target, in this case using a human for range finding. The markings assume that an average human is 1.7 meters (5 feet 8 inches) tall. The user simply aligns the scope so that the person's feet touch the bottom horizontal line and see which marking the person's head touches, as shown in the image below:

Public domain image

In this case, the person's head touches the 4 mark, which means that the person is about 400 meters away. Using this set of markings, the user can determine ranges from 200 to 1000 meters.

The PSO-1 scope has a bullet drop compensator (BDC) to adjust range in 50 meter increments from 100 meter to 1000 meter ranges. Therefore, once the range to the target is determined, the user can turn the knob and adjust the elevation to correspond to the appropriate range and then align the target with the top chevron mark (^).

Notice that the scope also has other chevron marks (^) below the first one. These are used to shoot at ranges beyond 1000 meters. The user sets the elevation to the maximum of 1000 meters, then uses the other chevrons to line up to 1100, 1200 and 1300 meters respectively.

Now we'll look at another reticle.

Reticle used by Schmidt and Bender scope. Public domain image.

The above reticle is used by scopes made by Schmidt & Bender. Note that the center of the scope has several dots in the horizontal and vertical lines. These are called mil-dots and each dot corresponds to 1 milliradian. Therefore, if the width or height of an object is known, the user can determine the range by counting the number of dots that it covers and then using the same distance formula that we saw above for the PSO scope. Therefore, if an average person, who is about 1.7 meters tall (or 5 feet 8 inches tall), covers 4 dots when viewed in the scope, we can take S = 1.7, mils = 4 and plug it into the formula D = 1.7/4 * 1000, which works out to 425 meters.

There is also another way to quickly compute the range with this reticle, without doing any arithmetic. Note that the bottom of the reticle, there is a long horizontal line and above it are a series of smaller horizontal lines in a step pattern. These lines can be used to quickly estimate the distance to a target, using a human as the scale. To estimate distances between 100 and 250 meters, the user simply frames the target's head between the lines as shown below:

Public domain image.

The average human head is around 0.25 meters high. The two lines that best frame the top of the helmet to the chin tell the distance to the target.

For longer ranges, the same horizontal lines can be used, except that the user frames the top of the target's head to the belt buckle between the two lines.

Public domain image

Using this, the user can measure distances between 400 and 1000 meters.

Note that in both these instances, the scopes are fixed power models. Therefore the user cannot adjust the magnification power and the rangefinding calculation is easier.

For scopes with variable power magnification, the method of rangefinding depends on whether the reticle is placed on the first focal plane (FFP) or second focal plane (SFP).

Recall in our last post, we mentioned that if the reticle is placed on the first focal plane, the size of the reticle resizes with the magnification, so if the user zooms into the target, the reticle also appears to enlarge in size correspondingly. So, if a target measures 4 mil dots at 3x zoom, it will still measure 4 mil dots at 10x zoom, when using a FFP reticle. Therefore, for a variable power scope using a FFP reticle, the range calculation formula is the same as that of the fixed power scope, D = S/mils * 1000.

We also mentioned in our last post, that in a reticle placed at SFP, the size of the reticle does not change with magnification power. Therefore, for a SFP scope, the mil-dot range estimation is calibrated accurately only at one particular magnification power, generally at the highest magnification power setting, or sometimes at the middle magnification power setting. Some SFP scopes have an index mark on the power ring, to show at which magnification power setting the mil dots are accurate (for instance, some manufacturers set it at 10x power, Bushnell generally sets theirs at 12x power). The formula for range estimation changes a bit in this case. Assume a SFP scope where the mil-dots are calibrated accurately at 10x magnification power. The distance formula for this scope is:

D = (S/mils) * (mag/10) * 1000

where
D = Distance to the target in meters
S = Width or height of the target in meters
mils = Number of markings covered by the target
mag = Magnification power of the scope.

As you can see, with a SFP scope, the range calculation is a bit harder to do because it depends on the magnification power setting on the scope. Therefore, some users usually set their scope at the power setting that it is calibrated at and leave it there, so that they don't have to do the extra math. For instance, in the above example, if the magnification power is set to 10x (i.e. the same magnification power that it was calibrated at), the formula simplifies to D = S/mils * 1000 (i.e.) the same formula as for a fixed power scope. Alternatively, if they change the magnification power, they change it to half or double of the calibration setting. For example, a Bushnell Elite 4200 6-24x40 variable power scope is calibrated at 12x magnification power, so if the user wants to change the magnification power, they usually select 6x or 24x, so that the range calculation can be modified by dividing or multiplying by 2. If the user chooses any other magnification power settings, the math becomes correspondingly harder.

This is why many military scopes use fixed power scopes, to reduce the amount of math calculations that the user has to do, and also avoid the chance that the user makes an incorrect calculation due to not paying attention to the magnification power setting of the scope.

All About Scopes - III

In our last couple of posts, we studied about different types of scopes and what they look like from the outside. In today's post, we will look at some of the stuff inside a scope. Specifically, we are going to study about a part called the reticle.

We actually dealt with reticles a little over four years ago, when we studied telescopic sights originally. A reticle is a device consisting of fine lines, which is embedded into a telescope and helps the user to line up a target precisely.

The classic image of a telescopic sight is a target centered around two crosshairs, such as the image above. This is usually what is shown in movies and TV shows. However, there are many different types of reticles, which we will study.

Different types of reticles. Public domain image.

Thanks to movies, most people are familiar with the Fine Crosshair type of reticle above. Fine crosshairs allow the user to see more of the target and do not block out much light. However, it is easier for the user to lose sight of the lines, especially in complex backgrounds. Thicker lines are more visible, but they block out more of the image and lose some precision. Therefore, modern telescopic sights use a mixture of both (i.e.) thicker lines on the outside and thinner lines closer to the middle. Examples of this would be the Duplex Crosshair, the Mil-Dot and the Modern Rangefinding reticle above. The thick lines allow the user to quickly figure out where the center of the reticle is and the thinner lines allow for precision aiming.

Back in the day, the crosshairs of reticles were made of  natural fibers, such as hair or spiderweb. Later on, they were made of thin wires (and many scopes still use wire crosshairs to this day, especially cheaper ones). The wires are mounted on the inside of the telescope tube. By flattening the wire in different places, the manufacturer can make Duplex Crosshairs or Target Dot type crosshairs. The nice thing about wire reticles is that they don't block out much light and are very durable.

Another technique to make the crosshair lines is to etch the lines onto a thin plate of glass, using a diamond cutter. The thin plate of glass is then mounted inside the scope. The etched lines allow for more complex crosshair shapes, including circles, lines that don't need to touch or have gaps in between. This allows them to have features such as estimating range and bullet drop (such as that seen in the Modern Rangefinding and the SVD type above). The etched lines block off a bit more light and the thin glass plate may reflect some of the light back instead of letting it through. Modern scopes usually coat the glass with special coatings designed to minimize the reflected light.

For aiming in low light conditions, many scopes have illuminated reticles. The illumination is usually provided by a few methods. The first is to use a bit of fiber optic cable to collect ambient light from the outside of the scope and deliver it inside to the reticle. Another technique is to use a battery powered LED to provide enough light to illuminate the reticle. While this method requires the user to carry a battery with the scope, it has the advantage that the user can usually adjust the brightness by turning a knob. The user may also be able to change the color of the backlight illumination, if the LED method is used. The third method, which is used in military scopes, such as the Trijicon ACOG, or the British SUSAT sight, is to use tritium, which is a mildly radioactive form of hydrogen, to provide illumination. The tritium slowly decays and emits light as it does so. The nice thing about this is that tritium glows for a long time and could last 11 years or more before the tritium tube needs to be replaced.

As you may have observed in movies, if the crosshairs are backlit, they are usually red, though some products use green or yellow. There is a good reason for this. Red happens to be the color that least interferes with the user's night vision.

Reticles may be mounted inside the telescope tubes in one of two spots: the first focal plane (FFP) or the second focal plane (SFP). For fixed power scopes, it doesn't make any difference which focal plane the reticle is mounted at, but it makes a difference for variable power scopes. If the reticle is mounted at the first focal plane, then the size of the reticle resizes with the target (i.e.) if the user adjusts the magnification to zoom into the target, the reticle also appears to enlarge in size and if the user adjusts the magnification to zoom out of the target, the reticle also appears in decrease in size correspondingly. If the reticle is mounted on the second focal plane (i.e. closer to the eyepiece), then the size of the reticle remains a constant, irrespective of the magnification power. Americans tend to prefer scopes with reticles mounted SFP and this is used in the majority of the scopes. Some high end European manufacturers make FFP scopes on request.

We will study more about the advantages and disadvantage of FFP and SFP scopes, when we study the topic of scopes and rangefinding tomorrow.

All About Scopes - II

In our last post, we looked at some basics of rifle scopes. We will continue our discussion in this post.

As we saw in our last post, there are mainly two types of scopes: the fixed power scope and the variable power scope. The big difference between these two is that the variable scope has adjustable magnification. 

We will now look at how these scopes are specified. Fixed power scopes are usually specified as two numbers separated by x. For instance: 4x32, 12x40 etc. So what do these two numbers mean? The first number is the magnification factor of the scope. Therefore, in a scope marked as "4x32", this means it magnifies the image 4x times (i.e.) the object appears 4 times larger when viewed through the scope, than if it was viewed using just the eye. So what is the second number mean? The second number is the diameter of the objective lens in millimeters. Therefore, in a scope marked as "4x32", this means the objective lens is 32 mm. in diameter. In many cases, the unit of measurement is specified, so instead of "4x32", it may be more clearly specified as "4x32 mm."

A Bushnell 10x40 Fixed Power Scope. Click on the image to enlarge.

In the above image, we have a fixed power 10x40 scope made by Bushnell. What this means is that it has a 10x magnification and the objective lens is 40 mm. in diameter.

Variable power scopes also have similar designations, except that they have three numbers. The first two numbers are separated by a hyphen (-) and the third number is separated by x. For instance: 4-16x42, 6-24x50 etc. The first two numbers indicate the range of magnification power of the scope. Therefore, in a scope marked as "4-16x42", this means that the magnification factor of this scope can be varied between 4x and 16x. The third number indicates the size of the objective lens in millimeters. Therefore, in a scope marked as "4-16x42", the objective lens is 42 mm. in diameter. As with the fixed scopes, sometimes the specification includes the unit of measurement as well, so instead of "4-16x42", it may be more clearly specified as "4-16x42 mm."

A variable power 4-16x42 variable power scope made by Nikon. Click on the image to enlarge.

In the above image, we have a Nikon model M-223 scope, which is a 4-16x42 mm. scope. This is the model we studied in our last post, when we were studying the different parts of a scope.

So, a 10x magnification is better than a 4x magnification, right? Not quite. It is true that the object appears a lot larger on a higher magnification scope, but you see less of the surrounding area through the scope. For instance, if you're looking at a herd of deer through a powerful scope, you can probably see the fur very clearly, but you will be unable to tell which particular deer you're looking at, because you can only see a part of a deer's body through the scope. Also, it is very easy to lose sight of a particular deer if it moves off a bit, because the powerful scope only shows a small area at a time. Bear in mind that with a 10x scope, the field of view of an object at 100 yards (90 meters) is about 2 feet (0.66 meters) diameter. With a lower powered scope, you may be able to see both the head and the body of the deer and can tell which one it is in the herd. 

Higher magnification also reduces the brightness of the image. For instance, if you have two scopes, a 4x40 and a 10x40. They both have the same size objective lens (40 mm.), but they have different magnification power 4x and 10x. The image seen through the 4x40 will be brighter than that seen through the 10x40. This has to do with the exit pupil, which we studied about in the last post. The 4x40 scope has an exit pupil of size 10 mm., whereas the 10x40 has an exit pupil of 4 mm.

A scope with higher magnification is useful against targets at a long distance, but not as useful against targets close by.

Therefore, for general purpose hunting, a scope with magnification in the range of 3x to 10x works fine for many hunters. Some use variable power scopes that work in this range (such as a 3-7x or a 3.5-10x scope), others are perfectly happy with a 4x or 6x fixed power scope, some even go for lower power, such as 1.5x or 3x, because they don't hunt at longer distances. For long distance shooting, scopes with magnification of 9x to 18x or so are used and anything more than that can only be used for shooting at targets that don't move.

For most soldiers, the US military have generally equipped them with fixed power scopes, because soldiers work in stressful environments and a fixed power scope saves them worrying about which magnification factor the scope is currently set at. Most military scopes have relatively low magnification, so that they are useful at ranges where combat usually occurs. The US Army, Air Force and Marines use the Trijicon TA31RCO ACOG sight, which uses a 4x32 fixed power scope. The scope has advanced features, such as dual illumination technology provided by fiber optics and tritium.

US Marine using his ACOG scope. Click on the image to enlarge. Public domain image.

Most other military forces also do the same thing for their soldiers. For example, Canada's soldiers are equipped with a C79 optical sight which is a 3.4x28 scope, British soldiers have a standard SUSAT L9A1 sight which has a 4x25.5 scope, Steyr AUG rifles (used by Austria and Australia) have a built-in 1.5x scope made by Swarovski (the same people that make luxury glass chandeliers and jewelry).

Canadian C79 Elcan sight. Click on the image to enlarge. Public domain image.

Snipers have also traditionally used fixed power scopes until recently. During World War II, German snipers used 4x fixed power scopes and US snipers used 8x scopes made by Unertl through World War II and the Korean war. By the Vietnam era, 10x fixed power Unertl scopes were in use by the US Marine snipers, although a variable power Redfield 3-9x scope was also tried out. The Unertl model MST-100 which is a 10x42 fixed power scope, remained in US Marines sniper service for quite a while (until about 2007 or so). The US Army snipers used the Leupold Ultra M3A 10x42 mm. scope or the Leupold Mk 4 LR/T M3 10x40 mm. scope until recently as well. In the recent years, US snipers have been experimenting with variable power scopes. For instance, US Marine snipers have been working with the Schmidt & Bender 3-12x50 mm. scope and the US Army snipers have been working with the Leupold Mk 4 3.5-10x40 mm., Leupold Mk 4 M1LR/T 8.5–25×50 mm. and Leupold Mk 4 6.5–20×50 mm. ER/T M5 scopes. Sandia National Labs also recently demonstrated the RAZAR (Rapid Adaptive Zoom for Assault Rifles) technology based on a request from the US military to develop a compact zoom rifle scope.

In our next post, we will look further into some of the technologies inside a scope.

All About Scopes - I

Many months ago, we had studied about rifle scopes briefly, when studying different types of sights. In today's post, we will cover the subject in a bit more detail.

There are two types of telescopes available to shooters:

1. Fixed Power Scope - These are simpler and have a fixed magnification factor.
2. Variable Power Scope - These are more complicated and allow the user to adjust the magnification, according to the distance that the target is from the rifle.

To understand more about these two types, let us first look at the main parts of a scope:

A scope made by Nikon.

1. Eyepiece
2. Ocular Lens
3. Exit Pupil
4. Power Ring
5. Windage Adjustment Control
6. Elevation Adjustment Control
7. Objective Lens
8. Eye Bell
9. Objective Bell
10. Parallax Compensation Control

In the above image, 1 is the eyepiece, which is the end of the scope that the user looks through. The eyepiece encloses a smaller lens, called the ocular lens (2), through which the user views the target. The eye piece generally has a focusing control at the end of the sight to obtain a sharp image of the target and the reticle.

The exit pupil (3) is the size of the column of light that comes through the eyepiece: the larger the exit pupil is, the brighter the image. The exit pupil size is defined as the diameter of the objective lens divided by the magnification power of the scope. So, if the diameter of the objective lens is (say) 40 mm. and the scope has 4x magnification, then the exit pupil is 10 mm. For variable power scopes, the magnification can be changed, for instance, from 4x to 10x. This means that, assuming you have the same 40 mm. diameter objective lens as above, the exit pupil will vary from 10 mm. to 4 mm. (i.e.) if you increase the magnification, it will decrease the exit pupil size and vice versa. A smaller exit pupil means the image will appear dimmer and a larger exit pupil means the image will appear brighter. 

The power ring (4) is a feature that is only found on variable power scopes. By turning the power ring, the user can change the magnification power of the scope. This feature is not found in a fixed power scope.

The windage adjustment control (5) allows the user to adjust the scope in the horizontal direction (left or right). The elevation adjustment control (6) allows the user to adjust the scope in the vertical direction (up or down).

The objective lens (7) is the large lens which is further away from the user. This lens concentrates the light that goes through the scope. Larger lenses let more light in and in general, the larger the lens, the higher the magnification power of the scope. Typically, the diameter of the larger lens is measured in millimeters.

The eye bell (8) encloses the eye piece and the objective bell (9) encloses the objective lens. 

Variable power scopes of higher quality have a parallax compensation control (10). Basically, parallax is an optical effect caused by the objective lens not being coincident with the reticle. Therefore, putting the eye at different points behind the ocular lens makes the reticle crosshairs appear on different points on the target, which could cause aiming errors. The parallax compensation control allows the user to adjust for the parallax effect.

Some scopes (both fixed and variable types) also have a brightness control for the scope's reticle, so that the crosshairs can be seen in low light conditions. Some high-end scopes also have a feature called Ballistic Drop Compensation (BDC) which allows the user to adjust for the effect of gravity acting on a bullet (i.e. the amount the bullet drops as it travels a certain distance horizontally).

In addition to all these, we must also define a term which we used above: magnification. This is the ratio of the size of the image as viewed through the scope, compared to if it was viewed by the naked eye. For instance, if the magnification factor is 4x, this means an object appears 4 times larger in the scope than if the object was seen without it.

In the next post, we will study some more details about scopes.

Why does a M16 have Tall Sights?

When we look at an M16 rifle (or its semi-automatic only civilian cousin, the AR-15), there is something noticeable about their sights:

A M16A2 and two M16A4s. Click on images to enlarge. Public domain images.

In all these examples, note that the sights are a good 2.5 inches from the top of the barrel. Why is this the case with the M16 family? We will study the reason in this post.

The first thing to notice in the M16 family of firearms is that the stock is in line with the center of the barrel. In most other rifles, the stock is a little below the line of the barrel and they have small sights mounted on top:

Click on images to enlarge. Public domain images.

The reason why the stock is below the barrel is for ergonomics. When the user rests his/her cheek on the comb of the stock. the user's eye is aligned just above the top of the barrel and therefore the sights can be placed right above the top of the barrel for such firearms.

However, for such stocks, when a bullet is fired, the recoil pushes back in the line of the barrel and the resistance offered by the user's shoulder is below the line of the barrel and this creates a rotational torque. We studied this effect when reading up on muzzle brakes and compensators earlier:

Here, A is the force acting because of the recoil and B is the force resisting the movement from the user's hand and shoulder on the grip and stock. Since A is higher than B, this causes the rotational torque C, which causes the front of the barrel to rise when shooting. This can cause a loss in accuracy, especially when firing rapidly.

In the M16 family of rifles, the rotational torque is minimized by placing the stock higher, so that forces A and B are around the same line with each other. This is called a "straight line layout" and is done in order to make the rifle easier to control, especially when firing in burst fire or automatic fire modes. One more nice feature of the straight line layout is that it allows the operating rod and buffer to run directly back into the stock and thus reduces the overall length of the rifle. However, with a straight line layout, the user cannot comfortably aim the rifle if the sights are just above the line of the barrel. Hence, the solution was to make the sights taller on the M16 family, so that the user can place their cheek on  the stock and still look through the sights comfortably.

By the way, the M16 isn't the only rifle that does this. The German FG42 and MG42 and the US M60 are three earlier examples that have tall sights as well:

FG42 and M60 machine guns. Click on images to enlarge. Public domain images.

In the above images, note that they all have a straight line layout. The FG42 was one of the first firearms to have this feature and came out during World War II. The straight line layout was later adopted by others, such as the M16 family, the Russian Dragunov rifle, the German Heckler and Koch HK36 and XM8, the Swiss SIG SG 510 etc.

A Swiss SIG SG 510. Click on image to enlarge. 

Licensed by user Rama under the Creative Commons Attribution-Share Alike 2.0 France license from wikipedia.org

With taller sights, the user can rest his/her cheek on the comb of the stock as usual and still aim comfortably through the sights. This is why all rifles with straight line layouts have taller sights.

Sights: Zeroing a sight

In the last few posts, we've studied various types of sights. Irrespective of the type of sight used, it must be adjusted so that it points to the spot where the bullet will impact when the trigger is pulled. Such an operation is called "zeroing". We will now study how to zero a weapon.

There are quite a few issues to consider when zeroing a weapon. We will study these first.

The first issue that must be realized is that a bullet fired from a firearm doesn't travel in a straight line. Instead, it travels in an arc and also drifts to one side. We discussed why this happens and also defined two terms called windage and elevation during our study of sight basics. Therefore, if a bullet strikes exactly the point we're aiming through the sights, from a distance of (say) 50 meters, it will definitely not strike the point that we're aiming at, from a distance of 100 meters (assuming the sights aren't adjusted for elevation and windage) because of the way the bullet moves. Hence, when we say that a firearm is zeroed, there must be a reference distance to the target, at which the sights are zeroed. Since different weapons have different shooting ranges, the reference zeroing distance depends on the type of weapon and the ranges it is typically expected to be used. For instance, pistols may be zeroed at 15 meters or 25 meter range, but an M16 rifle is typically zeroed at 200 or 300 meter range. Even though most pistols can be fired beyond 25 meters and the M16 can shoot well beyond 300 meters, the reference zeroing distance is the distance at which it is typically expected to be used under most conditions.

The second issue is to note is that different manufacturers of cartridges may make propellants which produce different amounts of energy and the profiles and weights of the bullets may be slightly different from manufacturer to manufacturer. These could also affect the path of travel of the bullet. Hence, when the firearm is zeroed, it is normally zeroed with the type and brand of ammunition that it is most expected to be used with.

The third issue that affects where the bullet strikes is based on the individual and the angle that he/she holds their head and peers through the sights, when aiming the weapon. For instance, many rifle shooters place their head so that the tip of their nose barely touches the rifle stock. Since different individuals have different shaped heads, therefore a weapon that has been zeroed for one individual will not necessarily strike the same point if another person is aiming the weapon.

The fourth issue is that as there is wear and tear in the firearm, it may shoot differently after some use, due to wear of the rifling, chamber etc. Rough usage may also misalign the sights. Hence, all weapons will need to reset their zero after some use.

With that said, we will discuss a procedure to zero the iron sights of an M16 rifle (or its civilian variant, the AR-15). The same basic principles can be used for zeroing any type of pistol or rifle for any type of sight.

For an M16 or AR-15, the US Army and US Marine Corps advocate the reference distance for zeroing the rifle to be 300 meters. The actual procedure is done at 25 meters though, using a target that has everything scaled by 1/12th actual size.

The M16 has aperture type iron sights, which we studied about previously. Both the front and rear sights are adjusted in discrete incremental amounts. Each time they are adjusted by one increment, a click sound is heard. Hence, adjustments are specified in number of click steps.

Initially, the user adjusts the sights so that the rear sight is using the larger aperture hole (the 0-2 aperture) and turns the windage knob so that the windage indicator is centered about the windage scale, as shown in the image below. The elevation knob in the rear sight is then set to the 8/3 setting (300 meter setting mark. We will discuss why this setting is labelled "8/3" at the end of this article.) and then clicked one click clockwise past it initially. This rear elevation knob will not be touched again until the adjustments are completed.

Next, the front sight post is adjusted. Note that the front sight is mounted on a flat disc with four notches cut on its edge. There is a spring loaded detent that enters one of these notches and locks the sight in place. To lower or raise the front sight post, one must depress the detent with a special tool (or a sharp object, such as a nail or a bullet tip) and then rotate the sight post clockwise or counterclockwise as needed. Each rotation of 90 degrees produces one click. The front sight is raised or lowered until the base of the front sight post is flush with the front sight well.

At this point, the rifle is said to be set at "mechanical zero" initially.

Next, the weapon is taken to the range and placed on a firm base, such as a bench rest. In the case of an M16, there is a special zeroing target sheet that can be placed at 25 meter distance away from the rifle. The target sheet is appropriately scaled to simulate a target at 300 meter range. The zeroing target sheet is shown below:

The target sheet has a black silhouette with a circle in it. This is the part that the user is supposed to shoot at. There are also eight circles around the black silhouette, each one topped with an arc with an arrow head on one side of the arc. These eight circles indicate which sight to adjust (front elevation or rear windage knob) and the arrows indicate the direction to turn the sight adjustment (clockwise or anticlockwise) to adjust the shot towards the center of the target. Notice that there is also a grid in the target sheet and there are numbers that are on the edge of the grid. These numbers tell the user how many clicks to turn the front sight elevation or rear sight windage knobs. The numbers on the X axis are for how many clicks to turn the rear sight windage knob and on the Y axis are for how many clicks to turn the front sight. On an M16, turning the front elevation sight by one click moves the impact point of the bullet up or down by one grid square. Turning the rear windage knob by three clicks moves the impact point of the bullet by one grid square left or right.

The user initially mounts the rifle on a bench rest (or tries to get as stable a shooting position as possible, using a prone shooting position, sandbags for support etc.) and aims for the center of the black silhouette and fires 3 shots. The user then walks up to the target and examines it and sees where the three bullet holes are and determines their average center. The user then looks at the grid and determines which sights to adjust and how many clicks to adjust them by. Let's say the bullets impacted 5 squares to the right and 2 squares down from the center of the target. Then the user knows from the grid markings that he has to adjust the front elevation sight by 2 clicks clockwise and rear windage knob by 15 clicks anticlockwise. Then the user walks back to the bench rest, turns the sight adjustment knobs appropriately and then shoots 3 more times. Then the user walks back and examines the three new holes and sees if they are hitting the center of the silhouette now. If not, the user again looks at the edge of the grid and determines which sights to adjust and by how much and repeats the procedure until the center of the silhouette is hit to the user's satisfaction.

Now the user turns the rear elevation knob in the rear sight one click counterclockwise, back to the 8/3 setting (the 300 meter setting mark. Remember that it was turned one click clockwise past the 8/3 setting initially). At this point, the rifle is fully zeroed at 300 meter range. To shoot at different ranges, the user now merely needs to turn the rear elevation knob to the appropriate range setting.

The following video shows how the procedure is done:

Of course, this procedure simulates the 300 meter range by using a scaled target at 25 meters. Hence, any small deviation at 25 meters may become more prominent at 300 meters. Therefore, some precision shooters adjust their sights as above, but then take their weapon out to an actual 300 meter range and test to make sure it shoots correctly there also and make minute adjustment changes as needed. Then they set the rear elevation knob to different range settings, such as 400 meters, 500 meters etc. and fine tune the settings appropriately. This way, any small errors will be minimized at longer ranges and negligible at shorter ranges.

Incidentally, this might be a good place to explain why the M16 rear elevation control says "8/3" for the 300 meter setting. If you look at that knob, its minimum setting corresponds to the "8/3" mark. The 3 means the setting for shooting at 0-300 meter range. Now, if the knob is turned 3 clicks, the next visible digit is 4, which is for shooting at approx. 400 meter range. Three more clicks brings the next digit 5 for shooting at approx. 500 meter ranges, the next digit is 6 for 600 meters, the next is 7 for 700 meters and then when the knob is turned further, it completes one full rotation and comes back to the "8/3" setting, which is now the setting for the 800 meter mark, the max. elevation setting on the M16A2. This explains why that mark is labelled "8/3". On M4 rifles, the corresponding knob is labelled "6/3" because the max. elevation setting on an M4 is for 600 meter range.

Since accuracy is affected by wear and tear, temperature, vibration and shock, rough usage (i.e. the sights get knocked out of alignment) etc., it is necessary to re-zero the weapon periodically. How often this is done depends on the user, or organization procedures. For example, different branches of military forces have different requirements on how often to zero weapons.

Sights: Reflex Sight

In the last post, we studied some details about laser sights. The next type of sight that we will study in this post is called the reflex sight.

When we studied telescopic sights, we noted that scopes have reticles (crosshairs) etched inside to indicate where the barrel is pointing to. Similarly, one of the key features of all laser sights is that they project light towards the target to show where the weapon's barrel is pointing to. Reflex sights also have reticles, like telescopic sights, but there is one important difference in how the reticle is displayed. Reflex sights feature a beam splitter or a dichroic mirror. The image of the target is combined with a reflected image of a reticle and projects the reticle image on top of the target image. The reflected reticle image can be illuminated if needed, thereby providing a clearer image of the reticle.

Image licensed under the GNU Free Documentation License version 1.2 or later. Original image by user Falcorian at en.wikipedia.org

In the above image, we see a reflex sight with a bright red dot projected on the image. A sight like this uses a small battery to produce the bright red spot and then projects it on to the image. There are several reflex sights like this, such as the Aimpoint series, EOTech etc. The brightness of the red dot can be increased or decreased to match the surrounding light.

One of the more common reflex sights being used these days is the ACOG (Advanced Combat Optical Gunsight) that is used by the US Army and the US Marine Corps. This type of sight is manufactured by Trijicon and comes in a variety of configurations.

Click image to enlarge. Public domain image

Different ACOG models have different magnification powers, different colors and shapes of reticles and some have features to allow for elevation. All ACOG models have an unusual feature for reflex sights in that they don't use batteries for the reticles. Instead, they use fiber optic cords for daytime and tritium lamps for night time conditions. We studied these two devices earlier when we studied how to make iron sights more visible.

Reflex sights can be used under a variety of light situations and allow quick target acquisition. The user can use both eyes to aim, when using a reflex sight. On the other hand, reflex sights add a bit of bulk to the weapon and affect its balance.

Sights: Laser Sight

We've spent the last few posts discussing iron sights and telescopic sights. In this post, we will study a new type of sight that is mainly seen with handguns and rifles, the laser sight.

A laser is merely a light source which has the property that the light emitted is focussed as a narrow beam with very little divergence. This is unlike a regular light bulb that emits light in many directions in a variety of frequencies. Due to the property of low divergence of the beam, lasers can be used to indicate the aimed point on a target.

The original laser devices were extremely large and bulky, but due to advancement of semiconductor technology, they can be made much smaller these days. The typical laser sight today is small enough to be mounted to the underside or top of a pistol.

The laser sight is attached so that it is parallel to the barrel. Since a laser beam does not diverge much, the user can move the barrel until the light spot from the laser hits the desired target. The spot indicates the area where the barrel is pointing to. It must be remembered though, that the light travels in a straight line, but real bullets travel in an arc, due to forces of gravity, wind etc. The user may therefore need to allow for windage and elevation depending on the distance between the weapon and the target.

Most laser sights utilize red laser diodes. In the late 1990s, green laser diodes were invented, but it wasn't until 2007 that the first mass produced green laser sight was invented. Green laser sights are a bit more expensive than red laser sights and consume more battery power. However, green laser light is much more visible to human eyes than red laser light, especially in bright daylight conditions.

The two pictures above show a red laser and a green laser during day time and night time. In the bright day time shot, the red laser dot is barely visible in the picture, whereas the green dot is easily seen. The green dot is also seen much more clearly at night time, where the entire beam is clearly visible, not just the dot. With red lasers, only the dot is visible normally and the beam is usually only discernible when the surroundings have smoke or dust. This makes weapons using green lasers much more easier to aim than red laser sights.

One of the disadvantages of red and green laser systems is that the target (and other people surrounding the target) can also see the dot and be aware that he or she is being targeted. Also, the visible beam gives away the position of the person pointing the weapon. Therefore, some systems use an infrared laser that is only visible to people wearing night vision devices. Unless the target is also wearing a night vision device, he or she is not aware of being targeted and doesn't know where the person aiming the weapon is either.

Laser sights allow the user to quickly acquire a target. With other types of sights, the user needs to concentrate on pointing the sights to the target and therefore loses focus of things that are surrounding the target. Laser sights allow the user to not lose details of the target's surroundings. It also allows one to aim the weapon without physically staring down the barrel, which allows for greater concealment.

On the other hand, laser sights need batteries and there is the chance of the batteries running out when they are needed. Red laser beams are hard to see in daylight or in clean surroundings. Green lasers are more visible in bright or clear conditions, but they chew through batteries much more quickly than red lasers and are also more expensive than red lasers. Laser sights also alter the balance of the weapon somewhat and rough usage can change the spot where the laser is pointing, which means it no longer points to where the barrel is pointing.

Sights: Telescopic Sight - II

In our previous post, we started our study of telescopic sights. We will continue our discussion of such sights in this post.

In our previous post, we saw how an user can use the scope to estimate the range to a target. Once the range is estimated, the user can adjust for windage and elevation. As a matter of fact, most telescopic sights have controls to adjust for these parameters. Since some scopes have variable magnification, these may have additional controls to zoom the view and adjust for parallax error. Some scopes may offer an illuminated reticle for low light conditions and have an additional knob to adjust the brightness. Some very high end models even come with built in laser range-finders for better accuracy.

Since very few weapons are designed with built-in telescopic sights, most of these are mounted separately. Most western military rifles are designed with a standard picatinny rail on them, to which various accessories (including a telescope) may be mounted.

M4 carbine with scope mounted on picatinny rail on top of the weapon. Click on image to enlarge.

In other weapons, people usually attach a scope base to the rifle and then add scope rings to hold the scope in place on top of the base.

There are some advantages to telescopic sights. In most iron sights (except aperture type iron sights), the user needs to focus on three points simultaneously, which is not easy to do. With a telescopic sight, all the user needs to do is focus the cross hairs of the reticle on to the target. With a magnified view, it allows for shooting at longer ranges. The shooting as well as target identification are more precise as well. They also provide much more accurate windage and elevation measurements than other systems we've studied so far. Most modern high quality scopes are also surprisingly durable and can take a fair bit of punishment.

There are also some disadvantages. The main one is cost. Telescopic sights are way more expensive than iron sights, though the cost has come down in recent years. These sights are also more bulkier than iron sights and definitely alter the balance of the weapon. Thirdly, they cannot withstand bad weather as easily as iron sights. Also, some weapons that eject cartridges from the top (such as lever action rifles) need to have the scope mounted slightly off center, so that it does not interfere with the ejected shells.

On heavy recoiling weapons, the scope rings need to be tightened very consistently, otherwise the scope will go out of alignment.

For a long time, most militaries would only equip snipers with telescopic sights, because of the high cost of a scope. However, as the cost of scopes has fallen in recent times, some militaries have started to equip some regular infantry with scopes as well.

Sights: Telescopic Sight - I

In the last few posts, we covered a lot of details about different types of iron sights. In the next series of posts, we will cover another type of sight, the telescopic sight.

One feature in common among different types of iron sights that we studied previously is that they do not perform any image magnification. Hence, if the user has bad eyesight or if the target is somewhat further away, they are less effective. The telescopic sight attempts to solve this problem.

The first telescope was invented by a German-Dutchman named Hans Lippershey in Netherlands in 1608. Later improvements were made by other users, including the famous Italian scientist, Galileo Galilei. Soon after this, telescopes were quickly co-opted for use in warfare, for tasks such as observing enemy formations, determining where artillery shells are falling, observing enemy ships etc. It is, therefore, very surprising to discover that telescopic sights weren't used in firearms for a very long time. In fact, the first mention of a telescope in a firearms sight occurred around 1835-1840, which is almost 230 years after the telescope was invented!

The first mention of telescopic sights was by John Chapman in the book The Improved American Rifle, published in 1844. The author mentions that he was a civil engineer by training and had given Morgan James of Utica, NY, the concepts and part of the design of a sight that James had built for him. The Chapman-James sight was the first known telescopic sight designed for firearms. Later improvements were made in 1855 by one William Malcolm of Syracuse, NY, who learned how to make telescopes from a telescope maker. Such sights were in use during the American Civil War. The first telescopic sight that actually worked well for practical use, was invented in 1880 by one August Fiedler from the town of Stronsdorf, Austria, who worked as a forestry commissioner of Prince Reuss. There were other improvements made by various parties and soon, an Austrian firm named Kahles started factory production, thereby becoming the oldest known manufacturer of rifle scopes. So it was close to the 1900s that the popularity of telescopic sights really started. The Kahles Company is still around as a division of the Swarovski group (the same people known for making Swarovski crystal jewelry and chandeliers) and still making quality rifle scopes.

Public domain image. Click to enlarge.

Telescopic sights are of two types: (a) fixed magnification and (b) variable magnification. Variable magnification scopes can change their magnification via a zoom control and can therefore adjust to varying ranges and light conditions.

Telescopic sights usually have reticles to make aiming more precise. The image below shows various types of reticles:

Public domain image. Click image to enlarge.

The classic reticle one sees in movies is generally the Fine Crosshair type shown above. It must be noted that while fine lines are suitable for precision aiming, they generally tend to get lost in complex backgrounds. Thicker lines are more visible against noisy backgrounds, but they lose some of the precision. Hence, modern scopes use a mixture of both, (i.e.) thicker lines towards the outside and thin lines towards the center of the scope. Example of this would be the Duplex Crosshair, the Mil-Dot and Modern Rangefinding reticles in the image above.

Most modern scopes also have ways to determine the distance to the target, so that the rifle may be suitably adjusted for elevation and windage. This is done by making a series of graduated markings on the reticles, as seen in the Mil-Dot, Modern Rangefinding and SVD Type reticle images above. Note: The SVD type reticle was originally designed for the Soviet Dragunov SVD sniper rifle. Some more examples of such markings are shown below.

Above image is licensed under GNU Free Documentation License 1.2 by Kosiarz-PL at wikipedia.org

Public domain image of Schmidt & Bender scope reticle

Such markings make it easy to estimate the range of a target if its height or width are roughly known in advance. For example, in the Schmidt & Bender scope, which is used by Dutch snipers, an object that is 1 meter tall or 1 meter wide at a distance of 1000 meters will appear to be exactly the width or height between two of the dots in the reticle image above. Therefore, the distance to a target is determined by the formula:

distance in meters = (known height or width of target / number of dots) * 1000

So, say the user is aiming the scope at a human target. Say that the human target stands about 3 dots tall, when viewed through the scope. Assuming that an average human is about 1.8 meters tall, then distance to the human target is estimated as (1.8 / 3) * 1000 = 600 meters.

Now, by knowing the distance to the target and knowing how his rifle performs at various distances, the user can adjust the elevation and windage of the weapon accordingly.

There is also an even quicker way to estimate distance, which does not involve any arithmetic. Notice that in the lower half of the reticle of the Schmidt & Bender scope image above, there is a horizontal line and above it are a series of shorter horizontal lines in a step formation. These lines can also be used to determine distance by using a human target as the scale, without doing any mental arithmetic. To estimate distances from 100-250 meters, the user merely frames the target's head between the horizontal lines as shown in the image below. The average human's head with helmet is approximately 0.25 meters high. When viewed through the scope, the two lines that best frame the target's head tells the user the approximate distance to the target.

Public domain image

To tell distances between 400 - 1000 meters, the user frames the target's upper body (i.e. the area between the head and belt-buckle) between the same horizontal lines and estimates the distance as follows:

Public domain image

Similarly, for the other range finding scope, it may be aimed at a target of known width, such as a tank, and the range may be easily looked up:

From the image above, one may say that the tank is roughly about 275 meters away.

Well that's a lot to absorb in one post. We will continue to discuss more about telescopic sights in the next post as well.

Sights: Iron Sights: Improving Visibility of Iron Sights

In the last few posts, we've studied various kinds of iron sights. Now, one of the desired properties of any type of sight, not just an iron sight, is that they should be easy to see, but should not be bright enough to blind the shooter. There are a few ways that this can be ensured. We will study the methods here.

Since sights should not be very shiny and reflective, particularly the front sight, there are a few ways to reduce glare from the sights. First, there are metal treatments such as parkerizing and ferritic nitrocarburizing, which we studied about previously. These treatments not only apply a protective finish to the weapon, but also reduce the reflective glare. Another way to do this would be to simply apply some non-reflective matte finish paint on the sights. Yet another way is to bead blast the sights or cut serrations on the surface to make them less reflective. Finally, some sights have a hood around them to shade the sights and reduce the glare.

However, there is another problem to consider too. The sights should not be dark enough that they merge in with the background. There should be sufficient contrast between the sights and the background, so that they can be picked up much easier. There are several ways to do this as well.

The first method is very simple: the manufacturer simply paints a different color on the front and rear sights to make them stand out more.

In this case, the manufacturer has painted around the square channel of the rear sight with a white paint and painted a white dot on the face of the front sight as well. In other cases, it may just be two dots on the rear sights and one dot on the front sight. Another variant is to put one rear dot in the center of the rear sight and another dot on the front sight. When the sights are lined up correctly, the two dots are aligned vertically, like the digit "8". Another version of this is to have a white vertical line painted in the middle of the rear sight and a dot on the front sight. When the sights are aligned correctly, it looks like a lowercase letter 'i'. Some manufacturers use different colors for the front and rear sights. For example, some people prefer to have the front sight as red or gold colored and the rear sights with white colored paint. Regardless of the method used, these colors provide contrast with the background and therefore make the sights easier to pick up. The image below illustrates the different types of what was just discussed.

Image based off an original image uploaded by user Stannerd on wikipedia.org. The original image was licensed under the GNU Free Documentation license version 1.2 and permission is granted to copy, distribute and/or modify this image under the GNU Free Documentation licence version 1.2 or later.

Another way to make the sights more visible, especially in low light conditions, is to have the sights contain luminescent glass tubes. The most common way to do this is to fill the glass tubes with tritium, a mildly radioactive isotope of hydrogen. Tritium is also used in some luminescent watch dials. The glass tubes are also coated on the inside with a phosphorescent material. As the tritium decays, it emits beta particles that strike the phosphor coating and emit a glow in green, red, blue, yellow, orange, purple or white color, depending on the type of phosphor coating material used. Typically, green, yellow and orange are usually used with weapons. The glow from these tubes is not visible in bright light, so some manufacturers paint additional rings around the glass tubes so that they can be picked up easily during daytime as well.

There is no battery power involved here, as the glow is created by the beta particles emitted by the decaying tritium gas. Since tritium decays very slowly, such tubes have a fairly long lifespan. In fact, one well known manufacturer (Trijicon) offers 12 year warranties for their green and yellow sights. The downside to these are that the price of these is relatively higher than some of the alternatives.

Another method is to use photo-luminescent paint. Unlike the tritium filled sights, which glow at all times, the photo-luminescent paint needs to be exposed to a bright light source first, whereupon it absorbs some energy. When the light source is removed, the paint emits the stored energy out slowly and glows green in the dark. A few minutes exposure to bright sunlight or ultraviolet light is enough to keep it glowing for several hours afterwards.

The nice thing about this method is that it costs a lot less than tritium filled tubes. An existing sight can be easily converted to a photo-luminescent sight by any owner, simply by applying some adhesive strips. The downside is that it needs to be exposed to light first to work and it automatically gets discharged after a few hours and needs to be re-exposed to light again to work.

Another method that is recently gaining popularity is to use fiber optic elements. Short pieces of fiber optic cord are affixed to the iron sights. Any ambient light falling on the sides of the fiber optic cord is concentrated at the tip of the cord. Therefore, the tip of the cord glows brighter than the surroundings and makes it easier to pick up during daylight.

Typical fiber optic sights come in green and red colors. These sights are now seen on handguns, shotguns, rifles, air guns etc. Some manufacturers make sights that combine fiber optic cords with tritium filled tubes, so that the sights may be effectively used in both day and night time.