Also, considering the diverse terrains that a soldier could be fighting in (Jungle in VN or, and much more particularly important, Desert!) all kinds of gunk can find its way into the chamber and cause a failure to go into battery. We've all seen the occasions when a particular brand of ammo won't chamber in a pistol and we smack the back of the slide lightly to close the breach - that's pretty much what the FA is on an AR. Civilians don't think about this sort of thing (they all need to take a look at Ski over at AK Operators Union and check some of their torture tests!)