I recently attended a very informative AR Marksman class, and I was asked to bring a variety of mags for demonstrations. The instructor grabbed my brand new FDE pmag and loaded a handful of rounds and explained that in a "panic" type situation where your are trying to clear a jam and you "tap-rack-bang"...when you "tap" the pmag.. it flings a round out into the chamber causing a jam. the army also teaches their soldiers to insert a mag, then give it a good smack to make sure its seated good, the pmag will also fling a round then so when you drop the bolt, you get a nasty jam.
Either situation resulting in a blood chilling "click" when you need it to go bang.
I was shocked myself, so when I tried to explain it to one of my buddies, i popped the bottom of the pmag to show him, and the round almost hit the ceiling (new mag, tight spring) when it ejected it. Ill try to record a video of it this weekend.
