When the Department of Defense first started buying AR-15s, they were clean, fast-firing, and accurate weapons popular with the airmen and Special Forces soldiers who carried them. But as the Army prepared to purchase them en masse, a hatred of the weapon by bureaucrats and red tape resulted in weapon changes that made the M16s less effective for thousands of troops in Vietnam.
During a lull in the fighting in the Citadel, a Marine takes time out to clean his M16 rifle.
(U.S. Marine Corps)
(A note on measurements in this article: Most of the historical data in this article came from when the Army still used inches when discussing weapon calibers. The most common measurements are .22-caliber, roughly equal to 5.56mm ammo used in M4s today and .30-caliber, which is basically 7.62mm, like that used by some U.S. sniper rifles. There is also a reference to a proposed .27-caliber, which would have been 6.86mm).
The AR-15 was a derivative of the AR-10, an infantry rifle designed by Eugene Stoner for an Army competition. The AR-10 lost to what would become the M14. But a top Army officer was interested in smaller caliber weapons, like the AR-10, and he met with Stoner.
Gen. Willard G. Wyman was commanding the Continental Army Command when he brought an old Army report to Stoner. The report from the 1928 Caliber Board had recommended that the Army switch from heavy rifle rounds, like the popular .30-cal, to something like .27-caliber. The pre-World War II Army even experimented with .276-caliber rifles, but troops carried Browning Automatic Rifles and M1 Garands into battle in 1941, both chambered for .30-caliber.
These heavier rounds are great for marksmen and long-distance engagements because they stay stable in flight for long distances, but they have a lethality problem. Rounds that are .30-caliber and larger remain stable through flight, but they often also remain stable when hitting water, which was often used as a stand-in during testing for human flesh.
If a round stays stable through human flesh, it has a decent chance of passing through the target. This gives the target a wound similar to being stabbed with a rapier. But if the round tumbles when it hits human flesh, it will impart its energy into the surrounding flesh, making a stab-like wound in addition to bursting cells and tissue for many inches (or even feet) in all directions.
That’s where the extreme internal bleeding and tissue damage from some gunshot wounds comes from. Wyman wanted Stoner to make a new version of the AR-10 that would use .22-caliber ammunition and maximize these effects. Ammunition of this size would also weigh less, allowing troops to carry more.
Stoner and his team got to work and developed the AR-15, redesigning the weapon around a commercially available .22-caliber round filled with a propellant known as IMR 4475 produced by Du Pont and used by Remington. The resulting early AR-15s were tested by the Army and reviewed by Air Force Gen. Curtis LeMay. The weapons did great in testing, and both services purchased limited quantities for troops headed to Vietnam.
Pvt. 1st Class Michael J. Mendoza (Piedmont, CA.) fires is M16 rifle into a suspected Viet Cong occupied area.
(U.S. Army Spec. 5 Robert C. Lafoon)
Approximately 104,000 rifles were shipped to Vietnam for use with the Air Force, airborne, and Special Forces units starting in 1963. They were so popular that infantrymen arriving in 1965 with other weapons began sending money home to get AR-15s for themselves. The Secretary of the Army forced the Army to take another look at it for worldwide deployment.
As the Army reviewed the weapon for general use once again, they demanded that the rifle be “militarized,” creating the M16. And the resulting rifle was held to performance metrics deliberately designed to benefit the M14 over the M16/AR-15.
These performance metrics demanded, among other things, that the rifle maintain the same level of high performance in all environments it may be used in, from Vietnam to the Arctic to the Sahara Desert; that it stay below certain chamber pressures; and that it maintain a consistent muzzle velocity of 3,250 fps.
A soldier with an M-14 watches as supplies are airdropped into Vietnam.
(Department of Defense)
It was these last two requirements that made Stoner’s original design suddenly problematic. The weapon, as designed, achieved 3,150 fps. To hit 3,250 fps required an increase in the amount of propellant, but increasing the propellant made the weapon exceed its allowed chamber pressures. Exceeding the pressure created serious, including mechanical failure.
But Remington had told civilian customers that the IMR 4475-equipped ammo did fire at 3,250 fps as is. The Army tests proved that was a lie.
There was a way around the problem: Changing the propellant. IMR 4475 burned extremely quickly. While all rifles require an explosion to propel the round out of the chamber, not all powders create that explosion at the same rate. Other propellants burned less quickly, allowing them to release enough energy for 3,250 fps over a longer time, staying below the required pressure limits and preventing mechanical failure.
The other change, seemingly never considered by the M14 lovers, was simply lowering the required muzzle velocity. After all, troops in Vietnam loved their 3,150-fps-capable AR-15s.
A first lieutenant stands with his M-16 in Vietnam.
(U.S. Army)
Instead, the Army stuck to the 3,250 fps requirement, and Remington and Du Pont pulled IMR 4475 from production. The Army turned to two slower-burning powders to make the weapon work, but that created a new issue. The powders created a lot more problems.
The new powders increased the cyclic rate of the weapon from 750 rounds per minute to about 1,000 while also increasing the span of time during each cycle where powder was burning. So, unlike with IMR 4475, the weapon’s gas port would open while the powder was still burning, allowing dirty, still-burning powder to enter the weapon’s gas tube.
This change, combined with an increase in the number of barrel twists from 12 to 14 and the addition of mechanical bolt closure devices, angered the Air Force. But the Army was in charge of the program by that point, and all new M16s would be manufactured to Army specifications and would use ball powder ammunition.
Pvt. 1st Class John Henson cleans his XM16E1 rifle while on an operation 30 miles west of Kontum, Vietnam.
(U.S. Army)
Rifle jams and failures skyrocketed, tripling in some tests. And rumors that M16s didn’t need to be cleaned, based on AR-15s firing cleaner propellants, created a catastrophe for infantrymen whose rifles jammed under fire, sometimes resulting in their deaths.
Many of these problems have been mitigated in the decades since, with new powders and internal components that reduced fouling and restored the balance between chamber pressure, muzzle velocity, and ballistics. Most importantly, troops were trained on how to properly maintain the rifle and were given the tools necessary to do so.