Sights: Iron Sights: Aperture Sights

In the last couple of posts, we studied the basics of sights, some details about iron sights and some details about open iron sights. In this post, we will study another type of iron sight, the aperture sight.

Recall that in open sights, the rear sight is simply a plate or a post with a notch cut in it. The notch is usually a V-shape, U-shape or a square shape. The front sight is merely a post or a bead. The main problem with them is that open sights hide parts of the target when aiming at it and it is slower to acquire a target with them.

An aperture sight is similar to an open sight, except that the rear sight is a ring with a hole cut through the middle of it. The front sight on an aperture sight is similar or identical to an open sight (i.e. it may be a ramp, post or a bead) or it may have a ring too. The user peeps through the rear sight ring and aligns the front sight to the target. Due to the way that the human eye focuses on the front sight and the target alone, the rear sight ring appears as a blurred or a ghost-like ring to the eye. Hence, such sights are also called peep sights or ghost ring sights.

Click image to enlarge

In the above picture, we have an AR-15 A2 rifle (the civilian version of an M16). The two sights are marked in the above picture. The front sight on the left is simply a post type sight. Notice that the rear sight is part of the carrying handle Here's a view of the carrying handle from the other side.

Click image to enlarge

Note the two knobs at the back of the weapon. The lower knob is used to adjust the range of the weapon (by controlling the elevation) and the top knob controls the windage. There is also one more interesting feature about this sight, as the picture below shows

In the above image, we have the actual rear sight piece alone. Note that there are actually two rings here. One has a smaller aperture than the other. The user can switch between one or the other by pressing the piece with a finger. The bigger hole allows the user to pick up targets easily and the smaller hole allows targeting with greater precision. The bigger hole is sometimes marked 0-2 to indicate it should be used for targets between 0 to 200 meters away.

This is what a sight picture through one of these sights would look like:

Because of the way that the eye focuses, the rear sight ring becomes an out of focus, ghostly ring and the front sight is sharp and focussed on the target.

The above image is of a sight picture from a Heckler and Koch MP-5 submachine gun. In this case, the front sight is also a ring with a post in the middle.

These sights share many of the properties of the open type iron sights. Like all iron sights, they are very simple to use. These sights may be fixed or adjustable type to adjust for range. They are relatively small and unobtrusive (though not as small as open sights) and don't affect the balance of the weapon much. Rain and fog don't affect these sights at all. Like the open sights, some of these sights may have tritium filled glass tubes, so that they can easily be picked up in the dark. They are also fairly cheap to manufacture.

Like all other iron sights, they have the disadvantage that they have no magnification or night vision enhancement and therefore depend on the user's eyesight alone.

These sights have one big advantage over open sights though. With open sights, there are three points that the human eye has to focus on simultaneously: the rear sight, the front sight and the target. Since these three points are at different distances from the eye, it is hard to focus on these simultaneously. In fact, only younger people with good eyesight can do this properly. Older people and people with far-sightedness find it much harder to do this correctly and hence it makes it harder to aim an open sight. With an aperture sight, there are only two focus points to handle: the front sight and the target. The eye simply centers around the rear sight ring automatically and it appears as a ghostly ring. This makes it much easier to aim the aperture sight quickly and easily.

This is why aperture sights became popular for several military rifles in the last few decades. They are the best all-round type of iron sight by far. They have been used with weapons like the M1 Garand rifle, the M16 family, some Lee-Enfield rifles e.g. the well known Enfield No. 4 SMLE rifle from World War II etc.

Aperture sights first started to gain popularity in the late 19th century for rifles. The early aperture sights came in two types, the tang type and ladder type. The difference between the two really has to do with where the rear sight is placed and adjusted. On a ladder type, the rear sight is on the barrel and can be folded down when not in use. The rear sight can be slid up and down the ladder to adjust for range. The tang type is mounted behind the action and is therefore closer to the user's eye. Tang sights could also be folded down and the aperture moved up and down for range. They could also be adjusted for windage. Many sights also had vernier scales or micrometers (screw gauge) attached, so that they could be adjusted more precisely. Such sights were often made by third party specialist manufacturers, such as Marble Arms Co., Lyman Gun Sight Co. etc., and often installed separately by the owner of the rifle. Sights like these were common in the wild west and used in historically significant rifles such as the Sharps rifle, Winchester model 1894 lever action rifle etc., which we've already dealt with in some previous posts.

The two images above are a ladder and a tang sight. The first one has a vernier scale attached to the side and the second one has screw gauges. The second one can be adjusted both horizontally and vertically to adjust for for windage and elevation. Both may be folded when not in use.

To illustrate the difference between open sights and aperture sights, take a look at the two images below:

These are two images of the same target taken from two different rifles at different distances. The first image is through an open sight of an AK-47 at 50 yards and the second is through an aperture sight of an AR-15 at 25 yards.

Aperture sights remained very popular for both military and hunting applications for a very long time, until reasonably priced good telescope sights became available. They are still available on many weapons to use as backup sights, in case the high tech sights fail.

Sights: Iron Sights: Open Type Sights

In our previous post, we studied some basics about iron sights. Now we will study one of the types of iron sights, the Open sight.

In an open type sight, the rear sight is merely a piece with a notch cut in the middle. The notch is typically V-shaped, square shaped or U-shaped. The front sight is usually some kind of ramp or post.

The above picture is an example of an open-type sight. It is typically made of steel or polymer. Note the rear sight is merely a small piece with a square notch in the center. The front sight is a small post type. In the above example, the front sight is fixed to the barrel, but the rear sight may be slid left or right using proper tools, to adjust the horizontal alignment of the sight. In the above example, the rear sight also has a white line surrounding the notch, to give it greater contrast from the surroundings, to make it easier to aim.

In some cases, the front and rear sights also have small dots of some material that glows in the dark (e.g.) glass tubes filled with tritium gas. This helps in picking up the sights in the dark as well.

Since tritium is a radioactive form of hydrogen, it slowly decays and emits light as it does so. Therefore, sights like these don't need any batteries and could easily last 10 years or more. These sights are much more visible in low light conditions.

These sights are characterized by their simplicity of design. There is not much that can go wrong with them. They are also small and unobtrusive, so they don't affect the balance of the pistol much at all. They also can't get caught easily on clothing or holsters because of their small size and shape. Hard blows may bend or move the sights out of alignment though, but on the whole they are pretty durable. Rain, fog and snow don't affect these sights much at all. These sights are cheap to manufacture and don't use batteries or electronics, so there is less to go wrong.

These sights are not as precise as other sights though. Square shaped notch on the rear sight provides the best accuracy of different open iron sight types. Other types are not as precise and also take longer to aim. Peering through the sights also obscures other details which are covered up by the sides of the sights.

Sights like these are pretty useful up to about 300 meters or so, though most average shooters can shoot them typically to 100 meters or so. These are therefore typically used by shorter range weapons such as civilian shotguns, police revolvers and pistols etc.

One famous assault rifle that was originally designed with open type sights is the AK-47. The rear sight is an adjustable one with a simple V-shaped notch cut on it, while the front sight is a post. The rear sight may be slid forwards and backwards to adjust for range. One may now purchase AK clones with other types of sights as well, but the original version was designed with open-type sights, which is typical for this simple, reliable weapon.

Sights: Iron Sights

In our previous post, we learned a bunch of basics about sights in general. The first type of sights we will study are called Iron Sights. The name itself is a bit of a misnomer these days, as this type of sight is rarely made of iron in modern times. Steel or polymer plastic are usually the materials of choice these days. However, in the early days of firearms, these were indeed made of iron and the name is a historical one.

So what is an iron sight? It consists of two pieces, the front sight, which is fastened towards the front of the barrel (muzzle) and the rear sight, which is fastened near the back of the barrel (the breech). These pieces may be fixed or adjustable to accommodate for range and windage. Aiming is done by using the human eye alone. There is no device for magnification (such as a telescope) or low light conditions (night vision device), just the two pieces. The user simply lines these two pieces onto the target and then pulls the trigger. The two pieces for most iron sights are in the shape of a bead, a post or a ring, or combinations of each (e.g. front sight may be a post and rear sight may be a ring, or both sights may be posts etc.)

There are two main categories of iron sights: (a) the open type and (b) the aperture type. We will study more about these types of sights in the following posts.

Iron sights are usually characterized by the following features:

• Simplicity: There is very little that can go wrong with iron sights, as they are very low tech indeed, compared to some other types of sights. They are mostly immune to recoil effects. Since iron sights don't have any electronics or batteries, there is less that can go wrong with them.
• Weather resistance: Iron sights are usually pretty sturdy and are not much affected by the effects of weather and temperature.
• Ease of training: These are pretty basic sights and it doesn't take a brain surgeon to figure out how to use them.
• Durability: Most sights are pretty resistant to shock, unlike other types such as telescopic sights. Since iron sights are usually smaller and low-profile, they are usually less likely to get knocked out of alignment when handling the gun. However, hard blows can sometimes knock even iron sights out of alignment.
• Less weight: Iron sights are very light and do not affect the balance of the gun much. This is unlike other types of sights, that may be much more bulky.
• Cheap: Since they are so low-tech, they are also very cheap to manufacture.

There are also some disadvantages with using iron sights:

• Lower precision: Not as precise as some other sight types and also difficult to adjust accurately.
• Lower range: Since there is no image magnification with iron sights, the aiming is based on how good the user's eyesight is.
• Reduced field of view: The front and rear sights will block out some of the surroundings when the user lines them up with the target, especially the lower half of the target.

Sights: Basics

In this next series of posts, we will talk about a topic that is very key to effectively using a firearm. If you can't aim a firearm properly, you can't hit the target you intend. The key to aiming a firearm properly is a good sighting device.

Most typical weapons have two sights: a front sight, usually closer to the muzzle of the weapon, and a rear sight, closer to the breech of the weapon. In order to aim the weapon correctly, the user aligns the weapon so that the two sights are in line with the target.

In some cases, the weapon may not actually have two dedicated sights, but some other part of the weapon may serve double duty as a sight. For example, in the classic Colt Paterson revolver design, there is a small blade on top of the barrel that serves as a front sight, but no separate rear sight. However when the hammer is cocked, there is a notch cut on the hammer, which is at the appropriate height to be used as a rear sight.

Early sights were mostly fixed sights, because the range of weapons was not that great. With the advent of firearms capable of firing at greater distances, firearm sights began to allow the user to adjust for range. Modern sights can be adjusted for both range (called elevation) and sideways movement (called windage). The image below is an example of an adjustable sight.

In this sight, there is a screw to move the sights horizontally left or right to allow for windage. The sight also can be folded flat or raised vertically and has multiple holes to accommodate different ranges. There is also an elevation adjustment screw to make minor range corrections. It is used as follows:

In the image above, the front sight is fixed and the rear sight is a folding adjustable type. If the soldier is firing at short range, he folds down the rear sight and aligns the folded rear sight and front sight with the target. Notice how the weapon is only tilted up slightly in this case. If he desires to fire at mid range or long range targets, he unfolds the rear sight vertically and looks through the appropriate hole in the rear sight and aligns the two sights to the target. Note that depending on the hole in the rear sight that he looks through, the front of the firearm is correspondingly tilted upwards and the range is adjusted for this way.

Modern sights also allow the user to adjust for windage. Basically, the rifling cut into a barrel makes bullets drift in a predictable manner. For instance, bullets fired from a given rifle may be known to drift between 4-5 cm. to the right, for a range between 200-250 meters. Therefore, to hit a target at a distance of 200-250 meters away, the user needs to aim about 4-5 cm. to the left to compensate for this drift. The effect of the wind blowing may also cause the bullet to drift a bit. By turning the windage knob appropriately, the user can adjust for the drift at different ranges.

Good sights should have glare reduction, so that they do not reflect too much light on the shooter's eyes in bright sunlight. On the other hand, they should not be so dark that it is hard to pick them up in low light conditions. For people like police officers and security guards, sights should not have sharp edges, in case they tangle up with clothes or gun holster, when pulling out the weapon. Good sights should also not lose their settings, once they have been adjusted appropriately.

In the next few posts, we will take a look at various types of sights and study their advantages and disadvantages.

Free Floating Barrels

In quite a few previous posts, we've made references to "free floating barrels", usually in the context of accuracy. Phrases like "disadvantage of this system is that it doesn't allow barrels to freely float" have been encountered in the previous months. So what is this "free floating" business and what does it do for accuracy of weapons. This post attempts to answer such questions.

First, let us recall our discussions about various types of stocks from a couple of months ago: wooden stocks, injection molded stocks etc. Wooden stocks have a problem of warping, due to humidity and temperature variations. Some other stock types are not vulnerable to humidity, but will also warp with temperature variations. As a result, they could cause the accuracy of the weapon to be affected. One way we saw to eliminate these issues, is to lay a bedding of a temperature-insensitive epoxy bedding material. The epoxy material ensures that the barrel contacts the entire stock at all points and this contact does not change with temperature variations. Therefore there is a constant pressure dynamic between the barrel and the stock throughout the length of the barrel, which also improves accuracy. Another method, which we mentioned in the post about bedding materials (but did not elaborate upon then) is to only attach the receiver mechanism and the parts behind it to the stock, and leave the barrel floating in the air, without touching the stock anywhere. This is what is meant by "free floating".

When a weapon is fired, there are vibrations induced in the steel barrel as the bullet travels down it. If the barrel is touching the stock (which may be made of wood, metal, plastic etc.) in a non-uniform manner, the barrel vibrations may be dampened differently from shot to shot, which changes the point where the bullet strikes. Depending on where the hand holds the stock and how loosely or tightly the hand grips the stock may also affect the vibration dampening characteristics and thereby the accuracy of the weapon. With a free-floated barrel, there is no contact between the barrel and anything else. Therefore, there is nothing that interferes with the natural harmonics of the steel barrel and the pressure dynamic is constant throughout the length of the barrel.

Another reason to free-float is because barrels tend to heat up when they are fired, which causes the barrel to expand. If the contact points of the barrel and the stock are not perfect (e.g. if no proper bedding is done), the barrel will bend slightly across the high contact points and therefore mess with the accuracy of the weapon.

A quick way to test if a barrel is free-floated or not, is to slide a piece of ordinary paper between the barrel and the stock. If the barrel is properly free-floated, the user will be able to slide the paper all the way down the length of the barrel. If the paper cannot be slid all the way, that means that some portion of the stock is contacting the barrel.

Simple way to test if the barrel is free floated properly.

The following video shows how to check if the barrel is really free-floating as advertised and how to fix problems:

Of course, free floating a barrel does not immediately mean that accuracy is improved. It usually works best with heavier, large caliber barrels. With lighter barrels, they tend to flex more when a shot is fired and since the barrel isn't touching anything else to support it, the angle of flexing may be more, which could cause the shot to go to a different point from where the user is aiming. Therefore, thinner barrels are more improved by glass bedding, rather than free floating the barrel. However, glass bedding is a more expensive process. In some cases, users glass bed the first few inches of barrel after the receiver and then leave the rest to free-float.

Free floated barrels are generally used for highly accurate weapons, such as match-grade rifles and weapons used by snipers.

Actions: Gas Operated: Recap

Let us now recap what we've learned about gas operated systems in the last few posts:

1. Gas operated systems work by utilizing some of the high-pressure gas generated by firing a cartridge, to operate their extraction, reloading and cocking mechanisms.
2. These are generally used for weapons like rifles and shotguns. For example, most modern self-loading and assault rifles designed during and after World War II use gas operation. Examples include: AK-47 and its descendants, FN FAL, M16 family, M1 rifle and carbine, Valmet, Galil etc.
3. The two main ways of using a gas operated system is to (a) make the gas act upon a piston, which drives the rest of the system or (b) make the gas directly impinge upon the bolt and bolt carrier.
4. Main problem with gas operated systems is that the gas tends to leave corrosive deposits on the working parts. Different gas operated systems handle this issue in different ways.
5. Another problem with gas operated systems is that the heat from the gas causes metal parts to expand. Due to unequal expansion rates of the various components, they must be well lubricated to prevent excess wear and tear.
6. Because of the weight of the parts moving after the weapon is fired, gas operated systems using pistons lose some accuracy because of the vibrations and shift in balance. This especially significant in the case of automatic firing weapons. This problem is reduced with direct impingement systems because the mass of moving parts is lesser than a piston driven system.
7. On the other hand, piston driven systems are generally more reliable because the gas only acts on the piston and is not allowed to directly act upon the rest of the mechanism. Direct impingement systems need to be cleaned and maintained more regularly because the gas acts upon much of the mechanism.

As we mentioned above, the hot high pressure gases generated by an exploding cartridge often carry corrosive chemicals with them. Upon cooling, these chemicals deposit upon the operating parts of the action. If the build up is too much, this will lead to jamming and reliability issues. All gas operated systems must therefore have some way to deal with this situation. For example, in piston driven systems, the gas is only allowed to act on a piston, thereby protecting the rest of the parts from heat and corrosion. Additionally, on an AK-47, the piston has sharp edges on its face, which in theory, scrape the built-up deposits from the walls of the cylinder. These scrapings get vented out the next time the weapon fires. In direct impingement systems such as the M-16, some parts such as the gas tube are somewhat self-cleaning and not designed to be user-serviced in the field. Other parts, such as the bolt and carrier are designed for easy access, so that they can be properly cleaned.

Most modern assault rifles and squad automatic weapons (SAW) in use today utilize the gas operated action principle. This shows the popularity and success of this type of mechanism.

Actions: Gas Operated: Gas Trap

In this post, we will study a lesser known gas-operated system known as the Gas Trap system. This was only used for a few rifles in history, but it might be well to mention it now. This system was originally invented in 1909 by Danish firearm designer, Soren Bang, and used in what is known as the Bang M1922 rifle. The Bang rifle was used by both Denmark and the United States.

A similar idea was also used in the early models of the American M1 Garand rifle, but was later dropped in favor of a long-stroke piston system, because of some problems with the gas trap mechanism. Many older M1 Garands were retrofitted to use long-stroke piston mechanisms and original rifles using gas trap actions are now very rare and are highly prized models indeed.

The third user of the Bang design was the German Gewehr 41, which was designed to the Wehrmacht's requirement that the rifle should be gas operated, but there should be no hole drilled in the barrel to tap the gases. With this very tight restriction to be met, both the manufacturers (Mauser and Walther) who submitted entries into the competition ended up using the Bang design. The Gewehr 41 was made by both Mauser and Walther and used the same caliber ammunition, but they had other different design features, to meet some of the other requirements of the Wehrmacht. The Mauser design was not as reliable as the Walther design, so the majority were made by Walther. However, the rifle is also very rare today and is very highly prized by collectors.

In all the previous gas-operated actions we've studied, there is a hole drilled into the barrel, near the breech (closed end of the barrel) or the middle of the barrel. The purpose of this hole is to tap some of the hot expanding gases leaving the rifle. This tapped gas is used to operate the mechanism that extracts the old cartridge, loads a new one and also cocks the weapon. In all the previous gas operated systems we studied, the hot expanding gases push the piston or bolt.

In a Gas Trap or Bang system, there is no hole drilled in the barrel. Instead, some of the gases are trapped by a ring-shaped cone at the end of the muzzle (the open end of the barrel). The gases expand into the gas trap and pull a piston (instead of pushing, like the other gas operated actions). Pulling this piston actuates a mechanism to extract the cartridge and reload the gun.

The idea behind this system is that the mechanism to extract and reload the cartridge starts to work only after the bullet has left the barrel. This enhances the accuracy of the weapon.

The problems with the system largely stem from the extra complexities of the gas trap. The gas trap is composed of some very precisely machined parts, which are prone to fouling and corrosion from dirty gases. This makes the system very hard to clean and maintain under normal operating conditions in the field and prone to jamming issues. In the case of the M1 Garand, there were also issues in trying to fit a bayonet on top of the gas trap. This is why the gas trap mechanism never really gained popularity.

Actions: Gas Operated: Direct Impingement

In our last two posts, we studied gas operated actions based upon pistons: the short-stroke piston action and the long-stroke piston action. Now we will study another modern gas-operated action, the direct impingement action. This action was originally invented in 1901, but the first successful rifle to use this action was the French MAS-40, which was commissioned in 1940. Since then, the most famous rifle to use this action is the American M16 family.

Recall that in the two piston driven systems we studied earlier, some of the gases generated by the burning propellant are tapped from the barrel and used to push a piston enclosed within a separate cylinder. The piston in turn acts upon the bolt carrier and bolt and causes them to cycle the rest of the action.

In a direct impingement system, there is no piston, piston rod or cylinder at all. Instead, some of the high-pressure gas is tapped out via a gas tube and piped directly onto the bolt and bolt carrier. The bolt and/or the bolt carrier are fitted tightly into the chamber of the weapon and act as a piston. In the case of the M16 family, the bolt has piston rings fitted to the back in order to provide the tight seal.

Click image to enlarge.

Image is taken from "US Army FM23-9, M16A1 Rifle and Rifle Marksmanship", which is in the public domain.

The bolt and bolt carrier move back due to the gas pressure. While moving backwards, the bolt extracts the fired cartridge case and ejects it via a side port. A return spring pushes the bolt and bolt carrier forward and the bolt picks up a new cartridge from the magazine on the way forward.

The following animation gives a good idea of how this mechanism works:

There are some advantages to such an action. Since there is no piston, piston rod or separate cylinder for the piston and piston rod, therefore the overall weight of the weapon is reduced. Fewer parts also means reduced costs of manufacturing. Since there are fewer and lighter moving parts on the weapon, balance does not shift as much and therefore the accuracy of the weapon is increased compared to piston driven weapons, especially in automatic mode. The gas tube is smaller and lesser mass than a cylinder containing a piston and hence it does not affect the natural resonance of the barrel as much, which also contributes to greater accuracy. Since the moving parts are in the same axis as the barrel of the rifle, this also helps performance when firing rapidly.

The disadvantages of this action are chiefly caused by the hot gases acting directly on most of the operating parts (bolt, bolt carrier, firing pin, springs, extractor etc.). The gases contain some dirty residues, which deposit on the bolt and bolt carrier and will cause reliability issues, if not cleaned frequently. This means that people need to spend more time cleaning and maintaining weapons with direct impingement actions. This also makes the weapon more sensitive to the quality of propellant used in the cartridges. The hot gases also cause the operating parts to expand. Since the parts are tightly fitted and expand at different rates, this could cause jamming and acceleration of wear and tear of the moving parts. Heat from the gases can change the physical properties (hardness, elasticity, brittleness etc.) of the metal parts and cause excess wear. The heat also evaporates the lubricant in the mechanism and this could cause the moving parts to jam up as well.

An infamous case of this is the early problems that the M16 rifles faced. When the original rifle was designed, the ammunition used for testing used a certain type of propellant (a nitrocellulose based powder). However, when the rifle was put into production and sent to the first combat units, the military found that Dupont Inc. could not produce the propellant in sufficient quantities to the specifications demanded. Hence, the ammunition was changed to use a different propellant (a mixture of nitroglycerine and nitrocellulose) which was easier to produce and met the pressure specifications. Unfortunately, this new propellant was dirtier and generated more residue than the one used in the development prototypes. Also to save manufacturing costs, the chrome-lining in the chamber, which was in the prototype design, was removed in the early production models. The early production models were also issued with no cleaning kits or maintenance instructions. These three issues caused many of the early M16s to jam unexpectedly and the rifle earned a very poor reputation during the Vietnam war. After a congressional investigation, the chrome lining was put back and cleaning kits and maintenance instructions were issued and the frequent jamming issues were resolved. While the reliability issues were solved and the M16 had better accuracy than most other competing rifles, the early bad reputation that was earned by the M16 took years to disperse.

Actions: Gas Operated: Long Stroke Piston

In our last post, we studied the gas operated, short stroke piston action. During this post, we will study the other gas operated action that uses pistons, the long stroke piston. This is a mechanism that was used by the American M1 Garand rifle (not the carbine, which uses a short stroke piston), the Soviet AK-47 family (and its various derivatives made in other countries, such as the Israeli Galil, Finnish Valmet etc.), the Belgian FN FAL assault rifle etc.

Like the short stroke piston we've already studied, when the cartridge is fired, hot high pressure gases are generated which push the bullet out of the barrel. Most of these hot gases also escape behind the bullet, but some of it is tapped out and used to operate the extraction and reloading mechanism to eject the spent cartridge and load the next cartridge.

Click to enlarge image

Original file licensed under GNU Free Documentation License version 1.2 by user Thuringius, on en.wikipedia.org

In the above image, (1) is the gas port from which a portion of the hot gases leaving the barrel are tapped. (2) is the head of the piston and (3) is the piston rod. (4) is the bolt that holds the cartridge in place in the breech and (5) is the bolt carrier that moves the bolt. (6) is the return spring.

Initially, the bolt holds the cartridge in place in the breech chamber. When the trigger is pulled, it releases the hammer (not shown in the image above), which strikes the base of the firing pin (not shown in the image above). The other end of the firing pin strikes the base of the cartridge, which detonates its primer. The primer in turn, causes the propellant of the cartridge to burn and generate hot, high pressure gases. The generated gases expand and push the bullet out of the barrel.

Click to enlarge image

Original file licensed under GNU Free Documentation License version 1.2 by user Thuringius, on en.wikipedia.org

As the bullet is pushed out of the barrel, some of the high pressure gases generated by the exploding cartridge are tapped via the port (1) and enter the tube containing the piston. The high pressure gases act upon the piston head (2) and push it backwards. Since the piston rod (3) is connected to bolt carrier (5), they move backwards, compressing the return spring (6). At the same time, the bolt carrier (5) also picks up bolt (4) and moves it backward at the same time. The bolt has an extraction pin that picks up the spent cartridge case and pulls it out of the breech. As the spent cartridge moves backwards, it is ejected out of the gun via a side port. The piston, bolt carrier and bolt continue to move backwards and re-cock the gun. Meanwhile the high pressure gases acting on the piston head (2) are dissipated via a vent hole in the tube. When the bolt carrier moves backwards to its maximum, the compressed return spring (6) pushes it forwards again. This moves the piston, bolt and bolt carrier forward again and on the way forward, they pick up a new cartridge from the magazine and push it into the breech chamber. Now the weapon is ready to fire again.

In the above animation, you can observe clearly how the mechanism works (including the hammer and firing pin, which were not present in the images above).

The main difference between the long stroke piston action and the short stroke piston action is that in a short stroke action, the piston moves back violently only a short distance (usually less than its own diameter) and then it is stopped by a projection. The bolt carrier then separates from the piston and continues to move backwards due to momentum. As a result of this, a short stroke piston action generally taps gases from closer to the breech (closed end of the barrel). In a long stroke action, the piston moves a distance greater than the length of the cartridge. In fact, the piston stays attached to the bolt carrier throughout the operation. This causes the piston to have greater dwell time and therefore, long stroke piston weapons generally tap their gases closer to the front end of the barrel (the muzzle). It also means that the operating parts are longer and therefore heavier than that of a short stroke piston action.

The advantages of this type of action are many. Like the short stroke piston action, the hot, high pressure gases only act upon the piston head and therefore, the other operating parts (the bolt, bolt carrier, trigger assembly, hammer, firing pin, return spring etc.) are not exposed to the hot gases or the dirty residue at all. These weapons can therefore accept a wide variety of cartridges using different quality propellants, since the dirty residue from low-quality propellants does not have a chance to contact most of the key operating parts of this action. This means a more reliable action and ease of maintenance of the weapon. The extra weight of the piston rod acting on the bolt carrier means more positive extraction, chambering and locking of the cartridges.

There are also some disadvantages. The main one is that the mass of the moving parts alter the center of gravity of the weapon and thereby make it harder to keep the weapon pointed on target, especially when firing in automatic mode. The abrupt stops and starts of the bolt carrier, piston and bolt at either end of the cycle also contribute to the shaking. Also, because the operating parts are longer, the overall weight of weapons that use this action are a bit heavier than short stroke piston actions.

Actions: Gas Operated: Short Stroke Piston

The first gas operated mechanism we will study today is called the short stroke piston. This is the mechanism that is used in rifles such as the American M1 carbine and M14 rifle, the Belgian Fabrique Nationale SCAR assault rifle, the British SA80 assault rifle and the German Heckler & Koch G36 assault rifle.

When a gas operated weapon is fired, the cartridge generates hot gases, which push the bullet out of the barrel. Most of the gas also leaves the barrel behind the bullet, but a portion of the gases are tapped out and used to operate the extraction and reloading mechanism, to eject the old cartridge and load a new one.

The basic idea behind a short stroke piston mechanism is that when the cartridge is fired, some of the expanding gases are tapped by a port close to the firing point or the middle of the barrel. This means that the gas here is at relatively high pressure (compared to if the gas was tapped close to the open end of the barrel). This high pressure gas is then allowed to act upon a piston. The other end of the piston touches the bolt carrier of the rifle. When high pressure gas acts upon the piston face, it pushes the piston back very rapidly, which in turn pushes the bolt carrier back. The piston moves back for a very short distance (generally, a distance that is less than its own diameter) and is then abruptly stopped, either by a projection, or by a gas cut-off. The bolt carrier separates from the piston and continues to move back on its own, due to the kinetic energy imparted to it by the piston. The bolt carrier drags the bolt backwards, which extracts the fired cartridge out and ejects it via a side port. They continue moving backwards and cock the rifle again, ready to fire. When the bolt carrier reaches its most backward position, a return spring pushes it forward again. On its way forward, the bolt picks up a new cartridge from the magazine and pushes it into the firing chamber. As it reaches its forward most position, the bolt locks and the weapon is ready to fire again.

The animation above shows the mechanism of the LWRC short stroke piston mechanism. It only shows the movement of the piston and not the bolt-carrier or bolt, but you can get a good idea of how it works. Note how the hot gases are vented out close to the middle of the rifle.

There are some advantages of the short stroke piston system. Like all gas-operated piston systems, the hot gases only impinge on one side of the piston and the other end of the piston rod pushes the rest of the mechanism. This means that all the hot gases and carbon residue are kept away from the rest of the operating mechanism, which improves reliability and life of the operating parts. Also, since the piston is stopped very quickly, the total mass of recoiling parts is smaller and therefore this makes the weapon easier to control. The other advantage is that shorter barrels can be used with short stroke piston systems.

Actions: Gas Operation: Basics

In the last few posts, we've studied some modern firearm actions such as those that utilize the blowback principle and the recoil operated action. We will now study another family that is also heavily used in modern day weapons, the Gas Operated Action.

The basic principle of this family of actions is as follows: When a cartridge is fired, the propellant material burns and produces pressurized gas. The pressurized gas expands and pushes the bullet out of the front of the barrel. After the bullet is forced out of the barrel, the gas also leaves through the same exit. In a gas operated action, some of this pressurized gas is tapped out from the barrel via a port and used to operate a mechanism to remove the old cartridge, re-cock the weapon and chamber a new cartridge automatically.

The pressurized gas operates the mechanism in one of the following ways:

1. The pressurized gas operates upon a piston, which is forced to move by the expanding gases. The other end of the piston pushes the mechanism that ejects the old cartridge and loads a new one in. The two main types of piston actions are short and long stroke pistons. Piston operated mechanisms are used with weapons such as the M1 Garand, the AK-47 rifle family, the Bren LMG etc. Since the hot high-pressure gas operates upon the piston only, the rest of the mechanism is kept relatively cooler and cleaner, which makes the mechanism much more reliable to operate.
2. Gas trap: This works similar to long stroke piston mechanisms, but uses lower pressure gas to operate. This is a relatively rarer mechanism and was used with the German G41 rifle in World War 2, as well as some early Garands.
3. Direct impingement: Instead of the gas to push a piston, which in turn operates the mechanism, in a direct impingement action, the gas directly acts upon the bolt and carrier. Since there is no piston, this reduces the weight of the weapon and lowers the manufacturing cost. On the other hand, the firing mechanism becomes fouled more quickly and hot gases may also cause the different parts of the mechanism to expand at different rates and thereby lose accuracy. Lubricants in the mechanism also dries up and may cause malfunctions. This is the mechanism used by the M-16, AR-15 and M4 rifles.

Compared to blowback and recoil operated actions, which are used for lower powered pistols mostly, the gas operated action family is primarily used for automatic and semi-automatic rifles.

The first mention of a gas operated action was due to a Mexican General named Manuel Mondragon, who used it in a rifle design called the Mondragon rifle in 1887. It was the world's first semi-automatic and automatic rifle. Due to poor production facilities in Mexico at that time, he tried to get some US manufacturers to make the weapon, but could not garner enough interest at that time. However, he managed to convince the Swiss firm Schweizerische Industrie Gesellschaft (SIG) to manufacture this weapon for him. Excess stock of these weapons from SIG were bought up and used by Germany in World War I and World War II.

The next patent for a gas operated action was filed by the redoubtable American designer, John Moses Browning, who invented a gas operated action in 1889. Browning later licensed his design to Colt in 1892 and it was used to make weapons in a number of calibers.

In the next few days, we will study the various gas operated actions in some detail.

Apologies for Small Hiatus

As you may have noticed, I stopped posting for a little while. The reason was that I was selected to serve on a jury for the last few weeks and it was quite an involved court case. For non-American readers of this blog, in the US court system, members of the public are randomly picked to serve as potential jurors for court cases. I've been called for jury service before, but this is the first time that I was actually picked as a juror. The judge told us at the beginning of the trial to not post anything about the case in blogs/websites during the duration of the trial. As the trial involved firearms, I decided to take a little time away from posting, just in case I accidentally posted something that could cause a mistrial to be declared.

I'm happy to say that I've finished my jury obligations and therefore I'm free to post again.