While it's definately going to cool the barrel down, immersing it in water is certainly causing damage as I mentioned before.
Chances are it'll still be useable for that battle, but don't count on it after that. It may even screw it up so badly that it cracks when fired (and that's definately something in the BAD THINGS category).
As for sniping, why bother wasting the ammo on poor, hapless cadets, even if they are only blanks. I found waving a handaxe, or, better yet, charging with a SMLE bayonet on the end of my F88 did the trick quite nicely. (Yes, a hundred year old bayonet will fit after judicious application of no more than an allen key)
There's something to be said for cold steel.
