Shooting underwater

[Laubage]

I don't know if this has been discussed yet, but I've always been under the impression that non-piston AR's couldn't be shot underwater/wet because they'd blow up (ie HK416 vs M4 test).

But today I found this video of an M&P15... Just out of curiosity, is the HK video a scam? Can you actually shoot a non-piston AR that's wet? Discuss.

HK416 vs M4
M&P15 underwater

 

[TacticalJ]

I don't know about shooting anything underwater, but if it was in the water and then the water was removed it shouldn't be a problem. The problem is that water don't compress like air and firing with a barrel full of water would not be good, at least in my opinion. I've never done it, but I've seen torture tests and it doesn't seem to be a problem, as long as you drain some!