They are less loud than they are startling. Had to drive a screwed up primer out with a decapping pin and needed a hammer. Scared the shit out of me, even though I "knew" it could happen, and scarred my table.
I was in a friend's shop when he set one off accidentally. I was dicking around with something while he was reloading some rifle rounds. He's working along and out of the corner of my eye I see him struggling with the press. Then he's bearing down on the press handle with everything he's got and BANG!
Apparently a primer had gotten flipped sideways in the press-mounted priming thing. He'd partially set the sideways primer and decided the best course of action was to mash it flat because the brass/primer was stuck in the shell holder. Yeah- don't do that.
How do you remove a backwards or messed up primer?
I just assumed the only way to do it will set one off sometimes.