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With the XM16E1 the Pentagon had demanded a change in the cartridge's propellant from the coarse-grained Improved Military Rifle (IMR) to the finer grained "ball" powder, which increased the automatic firing rate but also increased fouling of the bore and consequently caused the rifle to jam. To correct this, a "forward assist" plunger device to close the bolt in case of a misfeed was added. Colt, the Air Force, the Marine Corps and Eugene Stoner all agreed this device needlessly complicated the rifle and added about $4.50 to its procurement cost with no real benefit, but after another three years the Army ordered 840,000 of this version on February 28th, 1967. The version became known as the M16A1.
[[Image:M-16 gas impingement system.JPG|none|M16 direct gas impingement system]]
Colt had oversold the M16's reliability during testing, to the point where they claimed it never had to be cleaned. While this may have been true with the original IMR powder the rifle was developed with, it was not the case with the more finely-grained, faster-burning ball powder that was considerably "dirtier". The [[direct impingement]] gas operating system used a tube along the top of the barrel that vented gas back into a 'piston' formed behind the bolt and the recess in the bolt carrier. When combustion gases reached this area, they drove the bolt carrier to the rear and then vented the excess gas out the ejection port and out between the gas key at the top of the bolt carrier and the gas tube from the port in the barrel. Conventional designs keep the piston above or below the barrel and vent excess gas at that point. The advantage of the Stoner system was that the 'piston' formed by the bolt and bolt carrier operated in a direct line and on the same axis as the bore of the rifle. This resulted in no adverse movement of the bore axis due to the movement of the bolt carrier assembly. The adverse result of this design is that when the propellant gas is traveling down the tube, it cools and condenses becoming solid carbon. This carbon builds up in the action of the gun quickly. The M-16 therefore requires frequent cleaning to remain reliable.
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