Now, I haven't actually dunked numerous guns, but it would seem to me that most of them would work after being submerged so long as rust hadn't set in. I mean we're not dealing with electronics.
I would think the weak link would be the ammo, since all bets are off if the bullets get waterlogged. Unless you're saying the Glock guys claim their guns are waterproof. I'd find that hard to believe since there's no hatch at the end of the barrel. Although I did see one at the range the other day with a periscope attached to the tac rail