M16 Operation and Functioning Cycle

In our last post, we studied the similarities and differences between the AR-15 and M16 trigger mechanisms. Today's post will be another educational movie courtesy of the US Army (your tax dollars at work!). We will study the methods of operating a M16 rifle and its functioning cycle.

Today's movie was produced sometime in the early 1960s and describes the operation of the M16 rifle, specifically the earlier models. It also shows the functioning cycle of operation of a M16 rifle (i.e.) Firing, Unlocking, Extracting, Ejecting, Cocking, Feeding, Chambering and Locking and describes in detail how each stage works.

Note that this movie is pretty old, it actually refers to the rifle as the XM16E1. This is the first model of the M16 family that was adopted by the US Army. The earlier M16 rifle model was first adopted by the US Air Force, but the US Army insisted on a forward assist lever to be added to the rifle and therefore, the Air Force model was called M16 and the US Army version was called XM16E1 and later renamed to the M16A1 model in 1967. Also, in the early days, M16 magazines only held 20 cartridges and this can be seen in the movie as well.

Happy viewing.

Trigger Mechanisms of the AR-15 and the M16

A little while back, we studied how the trigger mechanisms of the AR-15 and M16 rifles work, complete with a video with animation explaining the concepts.

In today's post, we will look at a couple of videos that show the actual parts, instead of a 3-D Solidworks animation.

First up, an explanation of the AR-15 trigger mechanism:

Next, we have another video, that discusses the trigger mechanism of a M16 and compares it to that of an AR-15

Note that the M16 trigger mechanism being discussed is the M16-A1 model, which is capable of firing in semi-auto (single shot) and full-auto modes (unlike the M16-A2 and A4 models, which can fire in semi-auto and 3-round burst modes only). The AR-15 is capable of semi-auto fire only.

Happy viewing.

The M16 Family III - The Clones

A couple of posts ago, we looked at various members of the M16 family. All of the models we have looked at so far, have been developed for the needs of the American military. In today's post, we will look at some more members of the M16 family that were developed by other countries.

In 1984, Canada decided to equip their military with a new rifle and wanted to go with the NATO standard cartridge 5.56x45 mm. In order to save development time and money, they decided to purchase the license to manufacture M16s of their own. A little while earlier, the US Marines were working towards improving the M16A1 model to the M16A2. A Canadian military liaison officer was also present while the Marines were working on improvements and he communicated some of these design changes back to Canada. As a result of this, the Canadian C7 rifle was developed with some features of the M16A1 and some of the M16A2. A Canadian company, Diemaco, was put in charge of manufacturing the new rifle.

C7 (top) and C7A1 (bottom) rifles

The C7 rifle has the stronger and longer stock, barrel with 1 turn in 7 inches twist rate, pistol grip and handguards of the M16A2, but retains the older rear sights of the M16A1, as well as the same firing modes of the M16A1 (single shot and fully automatic). One more major difference, which is not obvious, is that the C7 barrel is manufactured by the hammer-forging process.

The C7A1 incorporated some improvements to the C7, the main one being the replacement of the carrying handle, with a rail that allows the user to attach other devices here, such as iron sights or optics. This is similar to the M16A4 model, however the C7A1 preceded the adoption of the Picatinny rail by the US military, therefore the rail on the C7A1 is of slightly different dimensions than the now standard picatinny rails.

The C7 and C7A1 models are now being replaced by the C7A2 model in the Canadian military. The C7A2 has picatinny rails, in order to be standards compatible with all the accessories that are made to fit on a picatinny rail. It also has rails in front to attach laser pointers or lights. Unlike the previous models, it has a telescoping stock. One more major difference is that the C7A2 rifle furniture is green in color, instead of black. Other changes include ambidextrous magazine release and ambidextrous safety selector levers.

The C7A2 model rifle

The C7 family also comes with corresponding carbine variants, the C8, C8A1 and C8A2. The C7 and C8 families are used by the military forces of Canada, Netherlands, Denmark, United Kingdom (Special Forces mostly), Norway (Special Forces), Canadian Police etc. In 2005, Diemaco (the company that made the rifles in Canada) was bought out by Colt and is now known as Colt Canada.

Another manufacturer of M16 clones is Singapore's ST Kinetics, which produces the M16S1 for Singapore's military. The M16S1 is simply a M16A1 made under a license by ST Kinetics.

Another clone of the M16 family is the Chinese made Norinco CQ. Unlike the C7 family, the Norinco CQ is an unlicensed clone of the M16. While it uses the same 5.56x45 mm. cartridges of the M16, the Norinco CQ was never adopted by the Chinese military. However, they make two models for export, one capable of selecting between single shot and fully automatic fire for military sales and a semi-automatic only version for civilian markets.

Public domain image of a Norinco CQ rifle. Click on the image to enlarge.

It is pretty easy to tell a Norinco CQ apart from other members of the M16 family. First, notice the shape of the stock is completely different from the M16A1, M16A2, C7, C7A1 etc. Second, the handguards in front are also differently shaped than the other members of the M16 family. Thirdly, the pistol grip is curved on the Norinco CQ. Fourthly, the front sight on a Norinco CQ is of the hooded sight type, instead of a sight post.

The barrel of a Norinco CQ has a 1 turn in 12 inches twist rate. Because of this twist rate, it will fire the M193 cartridge designed for the M16A1 properly, but cannot accurately fire the NATO standard SS109 or the newer US M855 cartridge beyond about 100 meters or so (because these require a twist rate of 1:7 for stability in the air).

While the military version of the Norinco CQ was never used by the Chinese military, it is used by military forces of some other countries, such as Cambodia, Syria, Libya, Iran (which makes their own copies of the Norinco CQ), Sudan etc.

Performance considerations for the M16 Family

In our last post about the M16 family, there was an interesting comment by a long time reader of this blog, Mr. Wojciech Imbierowicz. He questioned the ballistic performance of bullets on the M4 rifle, as compared to the M16A2. That will be the subject of today's post.

The first thing we will discuss is the relationship of bullet velocity to barrel length. When a cartridge is fired, the pressure of the expanding gases in the barrel is responsible for accelerating the bullet. As soon as the bullet leaves the front end of the barrel, the gases stop acting upon it. Therefore, the maximum velocity of the bullet is at the tip of the barrel and it starts to decelerate after it leaves the barrel. Obviously, if we have a shorter barrel, then the gases will not have a chance to accelerate the bullet much, before it exits the barrel. If we have a longer barrel, then the expanding gases will act on the bullet for a longer time and be able to accelerate it to higher velocities. Of course, there is a certain point in the barrel length, beyond which, increasing the length produces diminishing gains in the velocity of the bullet. Also, if a barrel is too short, the propellant may not entirely burn inside the barrel, which also reduces the force that is acting upon the bullet. On a longer barrel, the entire propellant may burn inside the barrel.

Depending on the type of cartridge, the optimum barrel length for maximum velocity may vary. The following table shows us some numbers for two common cartridges used by the M16 family, the M193 (the original cartridge that was used with the M16A1) where the bullet weighs 55 grains, and the SS109 (the NATO standard cartridge), where the bullet weighs 62 grains.

Barrel Length (in inches)	Velocity (M193) (Feet per second)	Velocity (SS109) (Feet per second)
10	2739	2627
11.5	2872	2738
14.5	3064	2907
16	3132	2989
20	3259	3095
24	3315	3158
26	3391	3231

As we can see from the above chart, barrels that are between 16 and 20 inches in length seems to be the sweet spot, after which, the increase in barrel length doesn't increase the velocity of the bullet by that much.

Now on to one more feature of the bullets used by the M16 family. The 5.56x45 mm. bullet is pretty small, but it has an interesting property that if it is travelling above a certain velocity when it hits a target, it tends to fragment and produce a wound much larger than the size of the bullet. Below this velocity, the bullet does not fragment reliably and produces a much smaller wound. For the M193 cartridge, this threshold is approximately 2700 feet/sec and for the SS109, it is approximately 2500 feet/sec. Of course, the material of the bullet and its construction also have a lot to do with how much velocity they need to fragment reliably. For the purposes of this discussion, we will only consider the two above-mentioned cartridges.

Now, from the table above, we see that if the barrel is 10 inches long, the velocity of the M193 bullet (2739 feet/sec) is just barely above its the fragmentation velocity (2700 feet/sec). The same is true for the SS109 bullet as well. Therefore, for both these cartridges, the bullet's velocity will fall below the fragmentation velocity pretty quickly beyond about 20 meters or so. For the record, the 10 inch barrel is used by some civilian and police forces AR-15 models.

With the 14.5 inch barrels (such as those used by the military's M4 carbine), we see that the velocities are somewhat higher (3064 feet/sec for M193 and 2907 feet/sec for SS109). This means that the bullets will travel about 100 meters before their velocities fall below the fragmentation threshold velocity.

Now, with the 20 inch barrels (such as those commonly used by AR-15s, M16A2 etc.), the velocities are a bit higher and therefore, the bullets can travel about 150-200 meters before they will stop fragmenting reliably. Therefore, between 100 to 200 meter distances, they generally have better wounding performance than bullets fired out of a M4 carbine, which only has a 14.5 inch long barrel.

To improve the performance, the US military has come up with some newer bullet designs that have lower fragmentation velocity thresholds. The newer M855A1 and the Mk 318 cartridges were specifically designed to handle some of the shortcomings of the previous cartridges.

The M16 Family - II

In our last post, we looked at some members of the M16 family. We also mentioned that the M16A2 model is a very influential model in the M16 family and is still in use in various military forces. We will continue to look at other members of the M16 family in this post.

M16A4: This rifle is the fourth generation rifle in the M16 series. One of the main differences between this model and the models we've have already studied so far is that the carrying handle on this model is detachable. What the designers did was put picatinny rails on the rifle and also on the hand guards. The carrying handle is attached to the picatinny rail on top and can be removed and replaced with other accessories, such as a reflex sight, night vision device etc. In addition, other accessories may also be attached to the other rail mounts, such as a laser, vertical grip, flash light etc.

The M16A4 rifle. Click on image to enlarge. Public Domain Image.

In the above image, note that the carrying handle has been replaced by a telescopic sight called an ACOG (Advanced Combat Optical Gunsight) and the rifle has an additional vertical grip attached to the hand guard in front. The M16A4 has the same firing modes as the M16A2 (i.e. single shot and 3 round burst) and also retains a bunch of other features from it.

The US Marines were the first to adopt this rifle model and are in the process of phasing out their existing M16A2 rifles with the M16A4. It is also used by some other countries' military forces around the world, such as Thailand, Turkey etc.

M4 Carbine: During the Vietnam war, Colt developed a carbine version of the M16A1, called the CAR-15. Around 1988, there came a need for another carbine design and Colt based it off the influential M16A2 design. The M4 carbine features a shorter 14.5 inch barrel and a telescoping stock. Therefore, it is more compact than the M16A2. It also borrows from the M16A4, by incorporating the rails and the detachable carrying handle, therefore allowing it to accept a wide variety of accessories.

US Soldier carrying a M4 with accessories attached. Click on image to enlarge. Public domain image

Due to its shortened barrel, the ballistic performance of the bullets is slightly worse than the M16A2. However, the smaller length makes it more suitable for close quarter battle. The M4 carbine was first selected for use in 1994, by the US Army, initially to replace the M9 pistol and the M3/M3A1 submachine gun. Later on, they began to replace the M16A2 rifles as well and the US Marines also adopted it for use by officers. Other branches of the military started using it as well and it is also used by other countries around the world.

One variant of the M4 is the M4A1 carbine, which is used by US Special Operations units, such as the Navy SEALs, Army Special Forces, Marines Force Recon, USAF Para-rescue etc. The difference is that while the M4 is capable of single shot and three round burst firing modes, the M4A1 is capable of single shot and fully automatic firing modes.

AR-15: The original prototype of the first M16 model was called the AR-15 (after the company Armalite, which was the original designer of this rifle family). However, after Colt bought the design, they called their model the M16 and made a civilian model of the design in 1963, which they named the AR-15. In this discussion, we will only talk about the civilian models released by Colt and other manufacturers, as few examples of the original Armalite design exist. The AR-15 was designed for civilian sales and while it does have a resemblance to other members of the M16 family, there are some very significant differences. The first major difference is that the AR-15 only has one firing mode (semiautomatic mode, which means you have to pull the trigger each time to shoot a new cartridge). Therefore, it is NOT an assault rifle, since it is incapable of switching firing modes. While the external appearance of an AR-15 is almost identical to a fully automatic M16 family model, the internals are very different. The hammer and trigger are of a different design and the bolt carrier and lower receiver are differently shaped that they cannot be interchanged with a M16 mechanism. This is done to satisfy United States firearm laws that semi-automatic weapons cannot be easily converted to fully-automatic weapons.

Colt AR-15 A3 Carbine Model. Click on image to enlarge.

There are multiple models of AR-15, resembling various other variants of the M16 family. While Colt officially holds the trademark to the term "AR-15", clones are made by many other manufacturers and they are unofficially all referred to as "AR-15". Many of these clones are designed to fire other calibers of ammunition, ranging from small pistol cartridges, to many different rifle calibers and even shotgun shells.

These are only some of the popular variants of the M16 family. There are other international variants that we will study in our next post.

The M16 Family - I

Many months ago, we had explored the differences between various members of the AK family (e.g.) AK-47, AKM, AK-74, AK-101 etc. In today's post, we will explore some of the popular members of the M16 family and discuss the differences between them. We will study the differences in multiple posts, as there are quite a few variants with major differences.

M16: This is the first operational version of the rifle and was only adopted by the US Air Force initially. Among its distinctive features are a triangular shaped forward hand guard, no cleaning compartment in the stock (as Colt famously declared that this rifle was so advanced that it didn't need one), no forward assist lever and a duckbill shaped flash suppressor at the muzzle. The original version was issued with a 20 round magazine. It is capable of selecting firing modes and can fire in single-shot as well as fully automatic modes. The barrel has a rifling twist of 1 turn in 12 inches.

Original M16 rifle. Click on image to enlarge. Public domain image.

In the above image, we see an original M16 rifle. This variant was adopted by the US Air Force, which continued to use this model until 2001 before switching to the M16A2 variant. It was also used by some other military forces, notably the British SAS.

M16A1: This is the variant that was first accepted by the US Army. Soon after the original M16 was ordered by the US Air Force, the Army got in the act and asked for a model with a few changes to it, notably, the addition of a forward assist lever, in case the cartridge did not properly seat in the chamber. Colt and the US Air Force believed that this change was unnecessary, over-complicated the weapon and increased its cost by $4.50. Hence, the Air Force got the M16 variant and the Army got the M16A1 variant., which has the forward assist lever. One more difference is that the duckbill flash suppressor was found to pick up twigs and leaves in jungle conditions and was replaced by a bird-cage style flash suppressor. This variant retains the same triangular forward hand guards, selectable firing modes (single-shot and fully automatic) and rifling twist of the original M16.

The M16A1 rifle. Note the bird-cage flash hider in front of the muzzle and the presence of a forward assist lever.

Early versions of the M16A1 were issued with no cleaning kits and without a chrome lined chamber. This led to a number of jamming issues early on and investigations were made to determine why this was happening. A change in the ammunition formula was found to be the cause, as the new ammunition left more residue in the rifle. Since the rifle was issued without a cleaning kit, soldiers could not properly maintain their weapons and this led to jamming problems in the field. Immediately after the cause was determined, the rifle was changed to include a cleaning kit, stored in a compartment in the stock. The barrel. bolt and chamber were also chromed to resist corrosion. In 1970, the magazine was changed from 20 round capacity to a 30 round capacity, as can be seen in the image above.

After its initial reliability problems were solved, the M16A1 variant became adopted by the US Army and the Marines, as well as other military forces around the world, such as Argentina, Australia, Thailand etc. It is still used by some military forces.

M16A2: This is a very influential version of the M16 family and has a number of differences from the M16A1. Most of the changes were added because of the US Marines. The Marines found that in the Vietnam war, many inexperienced men would put the rifle into fully automatic mode and end up emptying their magazines into the bushes without hitting a single enemy. Hence, they asked for the automatic firing mode to be removed and a three-round burst mode to be added instead. This makes the M16A2 variant capable of firing in single-shot and three round bursts only. Also, NATO forces wanted to use a more powerful version of the 5.56x45 mm. cartridge (the SS109 cartridge). However, the newer cartridge needed the rifling twist of the barrel to be changed, in order to maintain bullet stability in the air. Hence, the M16A2 features a barrel with a twist rate of 1 turn in 7 inches (compared to 1 turn in 12 inches for the earlier models) to work with NATO standard cartridges. The barrel thickness was also increased as the US Marines wanted a stronger barrel to resist bending in the field and not overheat as quickly.

The M16A2 rifle. Click on image to enlarge. Public domain image.

There are also many other major changes. For instance, the material of the buttstock plastic is different and 10 times stronger than the original. The shape of the forward hand guards is round instead of triangular and the two handguard pieces are symmetrical (meaning that either half can fit as the left or right side hand guard). This enables the rifle to be held more easily by smaller hands and the symmetrical hand guards make production easier, as there is no longer a need to manufacture separate left-side and right-side hand guards. The shape of the pistol grip is also different - this version has a notch added to support the middle finger and it has more texture to enhance the grip. Because this rifle is designed to fire the NATO SS109 cartridge, the rear sights are also changed to match this new cartridge's ballistics. Also, the flash suppressor is modified and closed at the bottom, so that it will not kick up dust or snow when fired from the prone position.

This variant was initially adopted by the US Marines and then by the US Army, followed by the other branches of the US military and other military forces around the world. It is still being used by many military forces around the world.

M16A3: This is a variant of the M16A2 model that came out at around the same time as the M16A2. It was produced in small quantities and designed for US Navy SEALS and the Seabee units. The main difference is that while the M16A2 can only fire in single-shot and three-round bursts, the M16A3 can fire in single-shot and fully automatic modes. Special forces are trained to not waste ammunition and hence, this variant allows fully automatic fire.

In the next post, we will study some more variants of the M16 family.