From what I have heard, one of the problems was that when DOD procured the M16, they started using ball ammo, which was left more carbon deposit in chamber than original 5.56 ammo than the original powder Armalite used in its AR15. One of the fixes was to chrome the chambers before the later model M16s were sent over to Nam (the new ones had a smooth-ended flash suppressor, instead of the old slot-ended ones that could be used to break the band on a case of C rations). I believe that the cyclic rate of fire was also slowed down somewhat in the later models. I carried a CAR-15 for a year and it worked fine (always kept it as clean as possible, and kept a piece of tape covering the muzzle to keep mud and twigs out, which would just get shot away with first round). Also always still carried a cleaning rod tape under the foregrip just in case I got a jam. Still quite a contrast to the AK47, AKM or SKS, which could be picked out of the mud and fired reliably without having to even clean it first.
