Will a gun fire underwater? — George, Tallahassee, Fla.
Marshall Brain Answers:
If it is a speargun, the answer is yes, as demonstrated here:
These guns are powered by rubber straps, making them more like slingshots than a traditional gun.
What about a normal handgun, shooting a normal gunpowder-powered bullet? The answer is yes again, as demonstrated in these videos:
The reason why it is possible to fire a gun underwater is because the bullet is self-contained. Gunpowder is an explosive that contains its own oxidizer. Therefore it needs no oxygen from the air. The bullet casing is waterproof, so the gunpowder and primer stay dry when submerged. Therefore, when the firing pin strikes the bullet, the bullet behaves exactly the same way it would above water.
What happens in the next millisecond, however, is different. The bullet is now having to push water, which is much heavier than air, out of the barrel. There is so much drag in water that the bullet loses all of its kinetic energy within a few feet. In the case of the shotgun in the video, the water provides so much resistive force that the gun cracks.
For more info on ballistic gel see: How Ballistic Gel Works – mayhem with fake flesh