The M16 has served as the United States’ primary service rifle for nearly half a century, and in that span of time, many variations of the rifle have been created. Some were prototypes that never went beyond the testing stages, others represented improvements to the original design, and some simply defy easy description.
While a complete history of all the unusual M16 versions could fill a book (and probably have), here is a look at three significant oddball M16 variants that reached production.
THE COLT 9MM SUBMACHINE GUN
The Colt Company has a long history with submachine guns dating back to the legendary Tommy Gun used by both gangsters and lawmen during the Prohibition era. But by the time of World War II, Colt was largely out of the submachine gun business.
This changed in the early when the company developed a submachine gun to compete with the popular Heckler & Koch MP5 in the lucrative law enforcement market.
Instead of designing a completely new firearm, Colt piggybacked on the success of the M16 by incorporating as much of the look and feel of the stalwart service rifle into the new design as possible. The resulting Colt 9mm Submachine Gun retained the characteristic M16 lower and upper receivers and operating controls.
The biggest changes were the elimination of the gas system in favor of a simpler direct blowback design and the caliber switch to 9x19mm. Like the MP5, the Colt SMG fired from a closed bolt, which contributed to its excellent reputation for accuracy.
The gun was adopted by many law enforcement agencies that liked its accuracy, reliability, and similarity to the M16. These agencies included the Drug Enforcement Agency (DEA), the U.S. Marshals Service, and the United States Marine Corps.
The original Model 635 had a 10.5-inch barrel with a 1-in-10-inch twist, a fixed carry handle with M16A1-style sights, and a M16A2-style flashhider.
The gun fired standard 9mm ammunition at a cyclic rate of 900 rounds per minute. The magazines were based on the Uzi design, and modified Uzi mags could also be used. Both fixed and collapsible stock versions were available.
The current models are the Model 991, which fires in semiauto or full auto, and the Model 992, which fires in semiauto or a three-round burst. These newer versions feature a flat top upper and quad rail for easy mounting of optics and other accessories.
The 10.5-inch barrel has a 1-in-10-inch twist. The rate of fire is listed as between 700 to 950 rounds per minute. These models weigh 6.7 pounds and are 26 inches long with the stock retracted, and 29.25 inches with the stock extended.
THE COLT M231 FIRING PORT WEAPON
Although the Colt M231 was one of the most produced, it was the least successful of all M16 variants. The M231 was designed to allow soldiers in the Bradley Fighting Vehicle to fire at the enemy through gun ports without leaving the protection of the vehicle.
A standard M16A1 could not be used because the Bradley’s six firing ports featured a screw-type mounting designed to maintain a seal against a chemical weapon attack.
The 231 was literally screwed into the fittings from inside the vehicle. The M231 shares about two-thirds of its parts with the parent M16A1 design.
As with the 9mm machine gun, the most significant change was the elimination of the M16’s gas impingement system in favor of a direct-blowback, striker-fired design.
This simpler mechanism is typically associated with smaller caliber submachine guns. Instead the M231 is chambered for the 55-grain version of the standard 5.56mm NATO cartridge.
The weapon weighs about 7.33 pounds, and has an overall length of 28¼ inches.
The barrel is 15.6 inches long with the same 1-in-12-inch rifling twist as the M16A1. In addition to being shorter than a M16A1 barrel, the M231 barrel also has a significantly thicker profile.
The shortened hand guards end at the distinctive metal locking collar. In use, the M231 has a generally poor reputation. The weapon fires full-auto only, with a cyclic rate of about 1,200 rounds per minute. The M231 does not have any sights.
The soldiers were to aim using periscopes mounted in the Bradley and spot their fire by firing only tracer rounds from standard 30-round M16 magazines.
The difficulty in aiming, combined with the extremely high rate of fire, meant that the magazines would be emptied before the shooter could get rounds on the target. Later modifications to the Bradley covered up the side firing ports with additional armor and now only the two firing ports on the rear hatch remain.
The original design included a simple wire stock so the M231 could be dismounted from the vehicle and used on foot. This stock was dropped from production models and Army procedures discouraged the use of the M231 outside of the vehicle.
Ironically, it is in this role that the M231 has probably seen the most use.
Photographs from Iraq show U.S. soldiers using the M231 as a backup weapon in vehicle turrets. They have also been carried by officers and armored crewman who normally are only armed with a handgun.
Since modern 5.56mm NATO ammunition is optimized for 1-in-7-inch twist barrels, and not for the older twist of the M231, the weapon is effective at only very short ranges. When you consider the lack of appropriate ammo, the absence of sights, and the difficulty controlling a weapon with such a high rate of fire, you understand how desperate a soldier has to be to use a M231.
THE COLT ADVANCED COMBAT RIFLE (ACR)
The Colt Advanced Combat Rifle was part of the Army’s search for a weapon to improve the average soldier’s ability to hit his enemy in combat. The specifications for the rifle called for a 100 percent hit probability increase over the thencurrent-issue M16A2 rifle.
The program started in the mid-’80s with six manufacturers submitting prototypes, and by 1989 only four remained. These included the Heckler & Koch G11, which fired revolutionary caseless ammunition, a pair of flechette weapons submitted by Steyr and AAI Corporation, and the Colt ACR.
The Colt entry was based on the then-standard M16A2 rifle with considerable upgrades. The full-length stock was replaced with the adjustable stock from the Colt carbine, the fixed carry handle was replaced with a rail that could accommodate an optic or iron sight, and a long rib was installed as part of the hand guards to serve as a simple sight for shotgunstyle point shooting. In addition, an oil-filled buffer and muzzle brake were installed to reduce felt recoil.
A special duplex cartridge was designed with two bullets loaded in each cartridge case to increase hit probability through better projectile dispersal. The Colt rifle could also still fire standard 5.56 NATO ammunition and used standard magazines.
The program ended in 1990 after extensive testing revealed that none of the candidates offered a significant enough advantage over the existing M16A2 to warrant replacing the standard service rifle. Although the Colt ACR was not adopted, several of its features are used in M16-series rifles and carbines today.
The flat top with a rail was later used in the M4 Carbine and M16A4, and the ELCAN sight used by the ACR is very similar to the current red dot sights that are now standard on both carbines and rifles in the U.S. military.
Story by Rob Reed