Here's an underwater test from an airsoft group with a long name, the Joint Operations Group GER Milsim Airsoft... "We like to play Hardcore Milsim and therefore our equipment needs to meet our requirements. So we took the Tippmann M4 underwater for a surprise assault training and it met our expectation! Don't try this with any other Airsoft gun, you will most certainly ruin it. If you want to play hardcore you have to go Tippmann!"
