There are surely different types, but the ones I've used have always fired one or two nails per activation of the compressor. You have to get the timing right to make sure you don't fire two. But after that, the gun needs more air so you have to squeeze again to get another nail out. It's not full-auto.
This sounds to me like a warning, not a suicide.
Unless it was an older powder .22 nail gun, those are semi and I have seen them get “stuck” on a burst.
Actually, the tip of nail guns must be pressed into something or modified with another nail or wedge of wood, which holds the tip in the right position to allow for rapid fire. There is no way this was self-inflicted, unless this guy could handle pain and love it at the same time.