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.

Stocks: Parts of a Stock

We will now study the parts of a gun stock.

The above image shows a typical English stock and an American one. The English stock was built with dimensions suited for about 80% of the sportsmen hunter population in England. Note that the English stock is built a bit more horizontal than the American one (2 inches displacement at the heel, vs. around 2.5" for the American stock). This is because American shooters generally tended to stand with their heads more erect and the same shooting style was also used in some other English colonies such as Australia and South Africa.

Public domain image from wikipedia.org

In the above image, 1 is the butt plate of the weapon (i.e.) the part that rests on the shoulder, 2 is the fore-end or fore-grip of the weapon. 3 is called the comb, which is the part that the user rests his/her cheek upon. The comb is built to support the user's cheek so that he/she can aim through the gun sights comfortably. Some combs also have a cheek-piece shaped on the side of the stock. 4 is the heel and 5 is the toe of the butt. This particular gun-stock, unlike the other two stocks above, has a grip (6) and a thumb-hole (7) for the user to put their hand through. Hence, this stock is called a thumbhole style stock.

The two types of stock are (a) one-piece stock (i.e.) the stock is composed entirely of one piece of wood and (b) the two-piece stock (i.e.) the butt is made of one piece of wood and the foregrip is made of another piece. The one-piece stock is generally used for bolt-action rifles, whereas the two-piece stock is used for assault-rifles, pump-action, lever action and break-open action weapons. Wooden one-piece stocks are usually more expensive to manufacture than wooden two-piece stocks, because it is harder to find a long piece of wood without blemishes, than it is to find two shorter pieces.

There are many factors important in shaping the stock.

1. Balance: This is the center of gravity of the firearm. If a thin thread is passed around the firearm stock, this would be the point where it would balance upon.

2. Cast-off or Cast-on: This is the amount that the stock is offset from the center when viewed laterally. A thin man may require a lot less cast-off than a fat man with stout shoulders, a right handed user may, due to a deformity, aim with his left eye etc. Usually, mass produced firearms don't have cast-offs, but custom-stocks may have cast-offs since these are produced for individual user needs. A cast-on is similar to a cast-off except it is done in the opposite direction for a left handed shooter.

3. Grip: The shape of the grip varies from stock to stock. Some may have the almost horizontal stock as shown in the English stock in the first picture of this article. Some may have a vertical grip like the thumbhole stock or a pistol grip, similar to that seen most modern assault rifles.

Stocks may be solid (i.e.) they do not change shape, or collapsible, such as a folding stock or a sliding stock. Sliding or folding stocks are generally features of military firearms.

In the above picture, we have an US made M4A1. Notice that the stock is two-piece, has a pistol grip and has an adjustable sliding stock at the back of the weapon, which allows the user to adjust the distance between the butt plate and the trigger to their own requirements.