This is a result of soldiers not being properly trained to begin with:
Washington, D.C., Monday, May 15, 1967
The subcommittee met at 10:15 a.m., in room 2216, Rayburn Building, Washington, D.C., Hon. Richard H. Ichord, Chairman of the subcommittee presiding. Other members are Hon. Speedy Long, of Louisiana, and the Hon. William Bray, of Indiana. The Chair inserted into the record the letter establishing and outlining the jurisdiction of this subcommittee:
There's a lot of other info on that site that has to do with the hearings on the M-16.But one of the young men did state that when the rifle first arrived that they had, I believe, a 25 percent incident of jamming, when the rifle first arrived in South Vietnam. He attributed the jamming to the failure of the military to provide them with proper cleaning equipment. He kept talking about not having a bore brush, that would get into the chamber mechanism in order to adequately clean the rifle.