Bump firing sometimes occurs by accident and probably has happened to you without knowing it. Ever touch off two rounds in quick succession even though you swear you didn't pull the trigger? Well, it could be soft primers or some mechanical wear but most likely it was a bump fire. The butt wasn't placed well enough into your shoulder and when the first round went down the range, the felt recoil bounced the stock against your shoulder and back against your trigger finger, launching another round. I've had it happen to me when I wear heavy clothes in the Winter and I'm pretty confident you've had it happen, too.
It's not pretty. It's also not accurate at all and not a good way to burn now-expensive ammo. It can be safe but as pointed out, you have a hard time hanging onto the muzzle.
Finally, there is actually a trigger attachment (BATF Approved btw) call the "hell-fire" trigger. It's actually designed to refine the bump fire idea with a spring loaded, adjustable trigger attachment. I've got one I've never tried nor will try. Frankly, once you shoot a "real" machinegun (I've been to Knob Creek twice and Westfield once) of just about any type, you lose your attraction to trying bumpfires on any semi. It's a pale imitation of the real thing.
You can, btw, bumpfire any semi-auto. It's not just for the AR.
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