the next bit, and a story...
What happens if you accidentally leave the BFA on when you switch to real, live ammo?
What happens then? You slap in the spare barrel and ROCK & ROLL!
When time permits, you take the first barrel to the small arms shop and have them check it over.
The BFA is a metal tube, with a small hole in it to bleed off the pressure, after it creates enough backpressure to cycle the action. This tube is held in position against the muzzle by a frame that clamps on to the front sight using a wingnut.
On a field exercise, my squad had the responsibility for an M60 position. Two men pulled duty in the hole, rotating every 4 hrs or so. After a week of this, the night before the exercise was scheduled to end, I got the midnight shift, and of course, that's when we were attacked.
While the agressors were tangled in our wire, my buddy and I engaged in a spirited whispered discussion as to whether or not we should shoot them. My buddy was in favor of just raising the alarm, because if we shot them, we'd have to clean the gun (blanks are notoriously filthy). I pointed out that we would have to clean the gun anyway, and we opened fire.
The gun was tripod mounted, and I was rapping off short bursts, with a good gunner's grip (one hand on the pistol grip, the other holding the feed cover closed). After about half a dozen bursts, there was a deafening noise, the sky lit up, and the gun kicked! Out in the wire a man screamed, and grabbed his leg.
One of them shouted "They got LIVE ROUNDS!"
I shouted back, "Damn strait, don't **** with us!"
The focus of the attack shifted and they gave our hole a wide berth.
I continued trying to fire the gun, but the action would only work manually. The blank adaptor had blown off.
The next 5 or 6 rounds (using the charging handle to cock the gun each time) only gave a pop and a small flash. (primer only) After that each shot was the normal sound and flash.
The next day, policing up the area, I carefully checked the fired blanks. One of them had the hole in the "nose" twice the size of all the others.
My best guess was that some bored troop took the powder out of several blanks, and crammed as much as he could into another.
The injured guy was taken to a med company, and I heard he would recover.
I have never seen live ammo fired with the BFA in place, but I expect something similar would happen, probably worse for the gun, and anyone unlucky enough to be hit.