This was a direct result of the gas tube becoming clogged by the calcium carbonate in the Ball powder.
Correct, but there were other reasons as well. The original 28 ArmaLite rifles were chambered in 223 Remington, as 5.56 didn't exist at this time, the cartridge went through an evolutionary process that resulted in what became the 5.56. Stoner designed the rifle around commercial 223 ammo which was loaded at the time with extruded propellant. The military wanted to switch to Ball powder for logistical reasons. They were already using it in M80 7.62 ammo and using it in 5.56 would simplify things for Lake City. And getting Ball powder to flow into a 22 caliber case necks was much easier than extruded on the high speed production equipment. The calcium carbonate didn't have any adverse effects on the weapons firing 7.62, and the fact that it took a few thousand rounds to cause problems in the AR didn't surface until the rifles were deployed in test batches to Vietnam.
The Ball powder caused another issue that didn't affect performance, but this illustrates how the military problem solves.
At the Colt factory, each rifle had to pass a series of inspections and tests before being accepted by the military. Each rifle had to for instance fire a high pressure test cartridge as proof of its integrity. There was an accuracy test. There was also a function test in semi and FA. And there was a ROF test in which the cyclic rate in FA was measured. It had to fall within a certain range.
The rifle was designed to pass this test with extruded powder. All was fine and dandy until the switch to Ball powder. Now the cyclic rate was too high and rifles were failing this test and therefore acceptance. Since this test was only conducted for initial acceptance and nowhere else after leaving the factory, the solution was to recall all the lots of ammo worldwide that were loaded with extruded powder and designate them only for the ROF acceptance test.
Fun fact:
The 5.56 was not standardized by NATO until 1980.
No cleaning equipment was issued with the rifle as with previous rifles. There was no equipment well in the stock for it either on the early rifles.
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So, this is what the 28 early protypes looked like. No caliber markings and slick sided magwell. Charging handle just like the AR10. Also note selector markings are not what we are used to seeing today. These rifles would go back to ArmaLite several times as the Army wanted changes made to the design.
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Very early Colt example with ArmaLite markings. Note 223 caliber designation and does not say US Property. This was probably a civilian model.
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This what the test batches (there was more than one) looked like that were sent to Vietnam for field trials. This is before formal adoption as the M16. The "XM" indicates experimental.
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