Before I retired from Uncle Sam 18 months ago, we had this occasionally happen with rebuild M16A2s that were all parts guns. Usually it was because the hammer-spring needed changing, OR occasionally (about 3 times a year) it was because the legs on the springs had not been properly installed on top of the lower receiver pins. As you know, the spring pressure from the legs of the internal spings is what holds the receiver thru-pins in place. Every now and then somebody would assemble a rifle with only one leg on the top of the pin or neither leg. This either made the spring sit canted, which caused problems, or it let the pin walk sideways after a few rounds. So the hammer (usually) would cant at an angle because the pin walked out on one side. When that happens (most of the time) it goes double or full auto a few times before finally jamming up. We had one guy who thought it was supposed to do that actually make expert on his rifle qual with a rifle that autoed every round in the mag.