Both M1 and M14/M1A rifles can fire out of battery even though the hammer is still held back by the sear. Extra thin/soft primer cups will fire the round by the inertia of the firing pin before the bolt's closed full into battery.
http://thefiringline.com/forums/showthread.php?t=532970
Search this forum for "slamfire M1A" and several links to discussions will show up. Member "Slamfire" has a lot of good info on such things.