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.

Picatinny Rails

When it comes to mounting accessories on to small arms, a picatinny rail is usually what is used to attach an accessory to the firearm in question. We will study this piece of technology in this post.

For many centuries, people developed many different accessories that could be attached to firearms, such as bayonets, telescopic sights, tactical lights etc. What was missing though was a standardized way to attach such accessories to a firearm. The US military decided to change all that and commissioned the Picatinny Arsenal in New Jersey to develop a standard for a mounting bracket that could be used to attach various devices. The Picatinny Arsenal developed a specification for a mounting rail called the MIL-STD-1913 rail, based on some work done by the A.R.M.S company back in the early 1980s. The specification was officially accepted by the US military on February 3rd 1995. This specification was also later adopted by NATO forces as the STANAG 2324 standard. Since the rail specification was developed by the Picatinny Arsenal, people often refer to the it as the "Picatinny rail".

Cross-sectional and side views of a picatinny rail. Click on images to enlarge. Images are in the public domain.

It consists of a rail with a number of raised ridges separated by spacing slots. The raised ridges are shaped like the letter 'T' in cross-section. Devices may be attached by sliding them on to the rail and then clamping them if needed with thumbscrews, bolts, levers etc.

Different models of picatinny rails made by various manufacturers. Click on images to enlarge 

Originally, the picatinny rail was meant to be used for mounting telescopic sights and was placed on the rifle's receiver, roughly where the rear sight should be. Nowadays, manufacturers place multiple rails on the firearm, so that many accessories may be attached simultaneously. Incidentally, these rails do not need to be attached to a rifle, many pistols can be fitted with them too.

HK-416 with multiple picatinny rails. Click on image to enlarge.

Image is licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license by user Dybdal at Wikipedia 

In the above image, we see a Heckler and Koch HK416N assault rifle used by the Norwegian army. This firearm has multiple picatinny rails on it, as can be seen in the image above. The user has attached a telescopic sight as well as a vertical foregrip accessory to the firearm and there's room to attach other accessories as well, such as a tactical light, laser aiming module etc.

Many rifles and handguns now come with picatinny rails on them. While the original purpose was to use it to mount telescopic sights, there are now manufacturers of other accessories, such as carrying handles, tactical lights, bayonets, bipods, laser lights etc., who make their accessories compatible with picatinny rails. Therefore, the same accessory can be easily mounted on to multiple firearms.

History and Development of the Assault Rifle - X

With the advent of bullpup designs in the 1970s such as the Austrian Steyr AUG and the French FAMAS, some of the newer designs that have emerged since then have largely been bullpup designs. Some of these include the British L85A1 and L85A2 of the 1980s, the Chinese QBZ-95, the German Heckler & Koch G11, Singapore's SAR-21, Israel's Tavor TAR-21 etc.

Since the advent of the 5.56x45 mm. NATO cartridge, the world has essentially gone into three basic calibers as far as militaries are concerned: the NATO 5.56x45 mm. (used by M16, Steyr AUG, FAMAS, L85, INSAS etc.), the Russian 5.45x39 mm. (used by AK74 family) and the Russian 7.62x39 mm. (used by AK-47, AKM and clones such as the Type 56). The first two of these are generally used by most military forces, whereas the third cartridge is generally widespread among many insurgent groups due to the popularity and wide-spread nature of the AK-47 and AKM family of assault rifles. However, there has been some other significant research in cartridge developments as well. Both the UK and the US did some work to find an intermediate cartridge between the NATO 5.56x45 mm. and the older NATO 7.62x51 mm., in order to strike a balance between bullet effectiveness and recoil force. This was due to the 7.62x51 mm. cartridge having too much power and weight and the 5.56x45 mm. not having sufficient range for some applications. Experiments showed that cartridges such as the 6x45 mm., the Grendel 6.5x38 mm. or the Remington SPC 6.8x43 mm. strike a pretty good balance by having more range than a 5.56x45 mm. cartridge, but still having relatively less recoil and less weight than a 7.62x51 mm. cartridge. With combat in Afghanistan taking place over longer ranges and the advent of scopes on assault rifles, it is possible for an infantryman to engage over longer ranges now and hence there have been noises made in various quarters to replace the M16 with a newer rifle using one of these cartridges instead.

Newer bullet designs using exotic technologies were also tried out during the 1970s and 80s, but they've been much less successful in this regard. For instance, there were attempts to design cartridges that fired flechette darts instead of conventional bullets. However, the cost of ammunition was prohibitive and thus this never became popular. Another exotic concept was folded ammunition, which was ammunition that was roughly U-shaped. The idea was to reduce the length of the cartridge in order to speed up the firing cycle of rifles.

Folded ammunition examples. Click on images to enlarge.

Folding ammunition never caught on either. Another concept was the caseless ammunition cartridge, for which a lot of work was done by Hecker & Koch and Dynamit industries. The idea is that since a caseless cartridge doesn't have a brass case, there is no need for the rifle to eject it after every shot, which reduces the number of steps in a firing cycle and thereby enables faster firing rates. H&K developed the G11 assault rifle to use caseless ammunition. The concept of a consumable cartridge is actually a very old one, as it was used by the Dreyse Needle Gun of 1835! However, with automatic weapons and modern ammunition, there are more problems to solve. For one, caseless cartridges are more easily damaged by rough handling since they don't have a hard outer case and two, the brass case removes some of the heat from the chamber of the firearm and the lack of a brass case means that caseless ammunition could cook-off in a hot chamber. H&K solved the first problem by putting cartridges in a sealed plastic case and had to spend a lot of time developing special propellants to solve the second issue. The G11 was about to be adopted by the West German military when the Berlin wall came down and the cold war ended. This resulted in cutbacks in military spending and H&K went into financial difficulties as a result and was acquired by the British, where they earned their keep by helping fix problems with the British L85 assault rifles. Interest in caseless ammunition technology has been renewed since 2004, due to the US military's Lightweight Small Arms Technology (LSAT) program.

Oddly enough, despite most of the new rifles being bullpup layouts, the US military is looking at conventional layouts for their M16 replacements. For instance, the US Army has switched to the M4 (which is the carbine form of the M16). The (now cancelled) XM8 project, which was designed to replace the M16, also had a conventional layout:

XM8 Assault Rifle. Click on image to enlarge. Public domain image.

The FN SCAR, adopted by the US Special Operations Command (SOCOM) as well as the US Marines M27 Infantry Automatic Rifle also use conventional layouts. Thus, it appears that the US is definitely not bucking the bullpup-layout trend, at least in the near future.

Another interesting concept developed in Australia is the Metal Storm rifle, where many bullets are stacked head to tail in series inside a barrel, with propellant between the bullets. Ignition of individual cartridges is accomplished electronically. Since the cartridges are completely consumable and because they are stacked one behind the other in line, there is no need for case ejection or a feed system to load new cartridges in the chamber. This makes the firing rate much faster than other designs, as well as contributing to reduced weight. As of now, Metal Storm products have found limited use and support from the US Marines.

What is the difference between a rifle, SLR, submachine gun, carbine etc.?

A very common question among newbies to firearms history is "What is the difference between a rifle, a sub machine gun, a machine gun, a carbine, SLR, assault rifle etc." This post attempts to point out the differences and name famous weapons of each type.

Rifle: The name "rifle" originally comes from the fact that the barrel of such weapons is "rifled". This means that the barrel has grooves in it to impart spin to the bullet as it comes out of the barrel. This spin helps to stabilize the bullet as it travels through the air and gives it a predictable drift (e.g.) a particular rifle may fire bullets that travel 1-2 cm. to the right every 100 meters on a windless day. This feature allows the user to reasonably predict where a bullet will hit and gives the rifle its accuracy. Earlier firearms, such as muskets, had smooth barrels. Smooth bore weapons were much easier to manufacture than rifles, but lacked in accuracy, as the bullets didn't have stability in the air and deviated in a random direction from the intended target. Therefore, the standard strategy for European armies of that period was to line up men in parallel lines and tell them to all simultaneously open fire at the enemy, because almost no one could reliably hit the enemy that he was actually aiming for. This massed fire strategy worked because some enemy would be hit, even if they weren't the ones that were originally targeted. These days, most modern firearms, even small ones, such as revolvers and pistols, have rifled barrels. Therefore, in modern times, the word "rifle" by itself, implies a firearm that has a rifled barrel, is designed to be fired from the shoulder, is manually fed (i.e. after each shot fired, the user has to manually operate a lever to feed a new cartridge into the weapon) and carries a small number of cartridges internally (say 1 to 5 cartridges). Examples of modern rifles from the early 20th century would be the famous American .30-06 M1903 rifle, the British 0.303 Lee-Enfield, the Mauser M98 etc.

Assault Rifle: In the early part of the 20th century, it was realized by some military forces that most conscripted troops, who were hastily trained in wartime conditions, had less than a 50% chance of hitting a target beyond 300 meters. It was also realized that most infantry encounters occurred at less than 500 meters and rate of firepower was also an important factor in these encounters. It was therefore reasoned that there was no need to give such soldiers a weapon and ammunition capable of hitting targets at 2000+ meter ranges, since most soldiers could not hope to hit a target that far away in the first place. Instead, it was reasoned that it was better to give troops lesser powered (and therefore, smaller and lighter) ammunition to suit the actual ranges that most infantry engagements occurred at and a correspondingly lighter weapon suited to fire this ammunition. With lighter ammunition and weapon, the soldier could carry more ammunition and therefore use a weapon with a higher rate of fire. While the concept was around before World War II, it was the German StG 44 (Sturmgewehr 44) that really popularized the concept. The word Sturmgewehr was coined by Adolf Hitler and literally translates to "Storm Rifle" (the word "storm" being used in the context of "storming a castle" or "assaulting a castle") and the term "assault rifle" is actually a translation of "Sturmgewehr". Even though there were earlier weapons that could be classified as assault rifles, many historians consider the StG 44 as the first to form the concept of modern assault rifles. In these times, an assault rifle is a weapon with the following characteristics:

• Has a rifled barrel (as in the case of the rifle described in the previous section).
• Has provision to be fired from the shoulder, just like a rifle.
• Uses an intermediate powered cartridge. This means that the cartridges are smaller than those used by a regular rifle, but larger than the cartridges used by a pistol. This is one of the key differences between an assault rifle and a regular rifle.
• Is capable of selective fire modes. This means it allows the user to select between firing different numbers of shots each time the trigger is pulled. In semi automatic mode, the weapon will shoot one bullet each time the trigger is pulled. The weapon automatically chambers the next cartridge to fire, but the user needs to let go of the trigger and pull it again to shoot it. In burst mode, the weapon will fire a preset number of cartridges (say 2 or 3) with each trigger pull. In fully automatic mode, the weapon will continue to keep firing as long as the trigger is pulled and there is ammunition. The ability to switch between various modes of fire is a key feature that distinguishes an assault rifle from other types.
• Has a detachable magazine. Most modern assault rifle magazines carry 20 to 30 cartridges or so.

Examples of assault rifles include the AK-47 family, the M16 family etc.

SLR: This is an acronym for "Self Loading Rifle". The term "SLR" is usually used in commonwealth countries. This has some common features with the assault rifle described above. Like the assault rifle, this is also a weapon that has a rifled barrel and is designed to be fired from the shoulder. It also uses an intermediate powered cartridge and has a detachable magazine like the assault rifle concept. The one key difference between assault rifles and SLRs is that SLRs have only one firing mode, the "semi-automatic mode". This means that each time the user pulls the trigger, the weapon will fire only one cartridge. It will automatically eject the fired cartridge, load the next cartridge and prepare the weapon to fire, but the user has to let go of the trigger and then pull it again to fire the next cartridge. This is unlike an assault rifle, which has multiple firing modes and allows the user to switch between them. Examples of SLRs would be weapons like the American AR-15 and the Indian Ishapore 1A1 SLR. The AR-15 is a semi-automatic version of the M16 rifle and the 1A1 is a semi-automatic version of the Belgian FN FAL assault rifle.

Carbine: This is a term used for weapons that are versions of rifles or assault rifles, but with a shorter barrel and lighter weight. Carbines are bigger than pistols, but smaller versions of rifles. In the days of the Wild West, mounted riders preferred a shorter and lighter firearm because these were easier to operate than full sized firearms, when riding. In more modern times, people inside moving vehicles or in close quarter jungle combat, preferred shorter weapons for the same reason. Since carbines have shorter barrels, they lack in accuracy compared to full sized rifles or assault rifles. While they use the same cartridge as their full sized cousins, the shorter barrel also means reduced velocity bullets. Many assault rifles also come in a carbine version. For example, the American M4 (a carbine version of the M16 assault rifle), the Israeli Galil SAR (a carbine version of the Galil assault rifle), Steyr AUG carbine ( based on Steyr AUG assault rifle. The carbine form uses a 16 inch barrel, whereas the assault rifle uses a 20 inch barrel) etc.

Submachine Gun: This is a weapon that shares some similarities with assault rifles, but one key difference is that submachine guns are designed to fire pistol cartridges instead of intermediate cartridges. As a result of this, submachine guns are generally lighter and smaller than assault rifles and are about the same dimensions or smaller than carbines. Since they use pistol ammunition, they also have less recoil and therefore can be fired from either the shoulder, the hip, or even holding it like a pistol. The first weapon to use the term "submachine gun" was the famous Tommy Gun that we studied about earlier. This is a fully automatic weapon that fires .45 ACP cartridges, the same cartridge used by the Colt M1911 pistol. Another famous submachine gun from the World War II era is the British Sten gun, which is also a fully automatic weapon. Modern submachine guns such as the Heckler and Koch MP-5 are capable of selecting multiple firing modes. Submachine guns are more preferred for close-range combat in urban environments because, with lower powered pistol cartridges, there is less risk of bullets penetrating through walls and hitting innocents on the other side

Machine Gun: This generally denotes a weapon that is capable of rapid, fully automatic fire and carries a large supply of ammunition. Fully automatic fire means that the weapon will keep firing as long as the trigger is pulled and there is a supply of ammunition available to it. Most machine guns fire either full sized or intermediate sized ammunition, similar to rifles and assault rifles. Machine guns generally carry a large supply of ammunition in ammunition belts, drum magazines or box magazines. This means that they generally have hundreds of cartridges available to them, unlike the 20-30 cartridge magazines used by assault rifles. They are also generally heavier than rifles and assault rifles. Examples of machine guns would be the Gatling Gun, the Gardner Gun, the Bira gun etc. Modern machine guns include the Browning Automatic Rifle (BAR), the British Bren gun, the American Stoner 63, the Belgian FN Minimi etc.

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.

Actions: Blowback Action: Roller Delayed Blowback

In our previous post, we studied the basics of the blowback action and learned a few things.

1. Blowback actions do not have the bolt locked in place at the moment of firing. Instead the bolt is held in place by inertia and springs.
2. When the cartridge is fired, it is desirable for the bolt mechanism to only start moving backwards after the bullet has left the front end of the barrel and the gas pressure in the chamber has dropped to a safe level. This is because: (a) a tight gas seal must be maintained until the bullet has left the barrel for greater range. It wouldn't do to leak gases out of the back end and reduce the pressure in the barrel while the bullet is still inside the barrel (b) It is not good for high pressure gas to blow through the magazine area and out the ejection port and rearrange everything inside the gun on the way out.

Therefore, there must be some method used to delay the backward movement of the bolt after the cartridge is fired.

As we saw in our previous post about straight blowback actions, one way to achieve this is to use a bolt that is much heavier than the bullet and use a stiff spring to hold the bolt in place. That way, when the high pressure gas acts on the bolt, it does not move much right away because of inertia. By the time the bolt and fired cartridge case start moving backwards, the bullet has already left the front of the barrel along with a lot of the high pressure gas, the pressure inside the chamber has already dropped to a much safer level and the bolt and cartridge case are moving back mostly due to momentum. Unfortunately, this makes the weapon much heavier than normal and harder to cock initially, as the user has to apply much more force to move a heavier bolt and stiff spring. Therefore, weapons that use straight blowback tend to use smaller and low powered cartridges to be suitable for practical usage by normal users.

Another alternative is to develop some kind of mechanism where the bolt encounters some higher resistance when initially trying to move backwards. This ensures that the bolt is delayed from moving backwards immediately after the cartridge fires and it doesn't move until the bullet has left the barrel. The idea is to put the bolt under some kind of mechanical disadvantage, so that much more force is required initially to move the bolt backwards. With such a mechanism, there is no need for a heavier bolt or stiffer spring and therefore the whole weapon can be lighter. Conversely, the weapon can also fire higher powered cartridges since the weight savings gained by a lighter bolt can be used to make the barrel heavier and breech mechanism stronger.

One way to do this is to use rollers to delay the bolt. Such an action is called roller delayed blowback. The most well known weapons that use this action are the Heckler & Koch G3 rifle and the Heckler & Koch MP-5 submachine gun.

The history of this type of action dates back to the middle of WW-II when it was used for an MG42 derivative and the Mauser StG 45. After the war, Mauser's factories were taken over by the French government and reorganized under a French organization called CEAM. Two of the former Mauser engineers, Dr. Ludwig Vorgrimler and Theodor Loffler were employed by CEAM and together perfected the roller delayed blowback design. Although they perfected the design based on the Mauser StG 45 cartridge (7.65 x 35 mm.), the French military then wanted to use a US designed .30 cartridge instead and so they were told to redesign for the new carbine. The French government also got involved in Vietnam by 1949 and therefore cancelled the project for lack of funding. Dr. Vorgrimler later left CEAM in 1950 and went on to join the Spanish company CETME, where they had already recruited some other former Mauser workers, as well as some others from another major German weapons manufacturer from world war II, Rheinmetall GmBH. Therefore, it was CETME that released the first commercial rifle using roller-delayed blowback, rather than CEAM. The post-war West German government was interested in the CETME design and licensed it for manufacture by Heckler & Koch and Rheinmetall. The result was the development of the Heckler & Koch G3 rifle. Dr. Vorgrimler was later employed by Heckler & Koch in the 1960s and went on to develop many more successful designs including the MP-5 submachine gun, which is still used by many forces worldwide.

The principle of the roller delayed blowback is to use two small masses (i.e. the rollers) usually attached to the bolt and arranged so that they move at nearly right angles relative to the bolt, at the moment of firing. The roller mechanisms act like levers and effectively multiply the mass of the bolt and bearings, which means that greater force is required to move them. The extra mechanical work that must be done to overcome this force generally allows the pressure in the barrel to fall to a lower level by the time the bolt starts to move backwards and thus delays the motion of the bolt until the bullet has left the barrel and the pressure has already dropped to a lower level.

The roller-delayed blowback mechanism of a Heckler&Koch G3 rifle. Public domain image.

US Patent 3283435. Drawing of roller-delayed blowback mechanism

When the weapon is cocked, the rollers (5a and 5b in the patent drawing) lock into the recesses in the trunnion (7a and 7b) and the face of the bolt head (4) sits flush against the base of the barrel, providing a tight seal. When the cartridge is fired, the bolt tries to move backwards and forces the two rollers (5a and 5b) against the locking piece (2). The locking piece, in turn, pushes against the bolt carrier (1) and the spring locking lever. The rollers act as levers and effectively multiply the mass of the bolt, which increases the force needed to push the bolt back. Once the rollers are pushed out of the trunnion recesses and are inserted back into the bolt head, the whole bolt assembly can now move backwards much easier. The bolt then moves back against spring pressure from the recoil spring (10) and the empty cartridge case can be extracted and ejected out of a port on the side of the weapon. When the bolt moves backwards to its maximum position, the recoil spring then pushes the mechanism forward whereupon, it picks up the next cartridge to fire from the magazine and loads it into the chamber.

The next figures from hkpro.com also make the concept very clear.

As the bolt head moves backwards, the force is transmitted from the bolt head to the rollers, as indicated by the horizontal red arrows. The rollers in turn, split the forces to the barrel extension and the locking piece, as indicated by the other red arrows. Approximately 75% of the force goes to the barrel extension and only 25% to the locking piece.

The extra force that is need to push the rollers out of the recesses and back into the bolt delays it enough for the bullet to leave the barrel and take most of the high pressure gases out of the front of the barrel. As a result, by the time the bolt starts moving backwards, the pressure of the gases in the chamber have dropped to a much lower level.

Image taken from hkpro.com. Unfortunately their web-server keeps going up and down, forcing me to not link to them directly.

In the above figure you can see the position of the rollers from where the bolt is in a locked and unlocked position. Once the rollers have moved out of the trunnion recesses, the whole bolt assembly moves back easily, just like in a straight blowback system; and the empty cartridge case moves back with it and is ejected out of the receiver.

There are some advantages and disadvantages of a roller delayed blow back system.

Advantages:

• The barrel can generally be free floated, since there aren't any moving parts up front. This is unlike a gas operated mechanism, where part of the mechanism is hanging off one side of the barrel and disturbing the natural harmonics of the barrel. Free floating the barrel increases the accuracy of the weapon.
• Unlike the straight blowback mechanism, this one can use higher powered ammunition.
• When used with suitable ammunition that is designed for the weapon, this is a pretty reliable design.
• Moderately cheap to manufacture.

Disadvantages:

• This design definitely doesn't like wide varieties of ammunition. It is sensitive to factors such as the weight of the bullet or the type of case. Therefore, the ammunition used for this weapon needs to be good quality and manufactured to uniform standards, otherwise it could cause operating issues.
• This is a design that can get dirty very quickly while using. It is fairly easy to clean up though.
• It is a relatively complex system compared to some other designs, such as the AK-47 mechanism.
• The locking delay can go down as the parts begin to wear out, chamber gets dirty or clean etc.
• Cannot be tuned by a user, unlike a gas-operated mechanism with a regulator is.

Though CETME invented the first commercial rifle using this action, Heckler & Koch are the best known users. Several Heckler & Koch products such as the G3 rifle, MP-5 submachine gun, P9 pistol, HK-21 and HK-23 machine guns, SL-6 and SL-7 hunting rifles all use this mechanism. Other users of this action include the Swiss SIG SG 510 rifle and CETME rifles such as the modelo A, B, C, L and LC .