IDescribe, I've done something similar at a week long rifle training class a few years back.
Lying on the shooting mat, on the firing line, M1A's, M1 Garands, a few AR15's spread out on both sides of me (about 25 centerfire rifles on the line all week). The range master is giving us instructions, I'm listening, making sure my magazines are where they should be, looking down range to ID my target, the range master is talking, I get my rifle "slung up", I insert the magazine and let the bolt go forward to chamber that first round, I'm looking through the sights, getting my NPOA and he shouts, "FIRE!" The safety goes OFF, my finger comes back to the trigger, I'm taking up the slack and I hear the strangest bleep loud noise. "Now what in the hell was that?" is going through my mind (I'm not in the trigger pulling mode now, I'm in the what the hell was that mode?) when I hear it again and suddenly I realize I don't have my hearing protection on. I have to drop my head and work my ear plugs on/in while shooting is going on on both sides of me. Then I'm ready to shoot.
It happened to me three times that week. In prone every time. Strangest thing is, it didn't hurt, my ears didn't ring then or later. It sounded (if you can believe 20 some center fire rifles going off all around you wasn't extremely loud) loud, but not like my mind thought 20 plus rifles should sound like.
Not sure why, and I didn't leave them out to see what would happen through the whole stage, but it was a strange experience/feeling to know that happened and it didn't really hurt.
Sometimes I think adrenaline can have some strange effects on the body.