Mythbusters

I was watching "Mythbusters" on TV and they demonstrated how a M1 (30 odd 6) can penetrate bullet proof glass. It also is known for going through car and truck engine blocks to stop vehicles. However, when they tested a .223 which I believe is a full metal jacket AR-15 or above, the full metal jacket did not penetrate the bullet proof glass and broke up into many many pieces when fired into water. So, likely if you were underwater and someone was shooting at you, you would want it to be an AR-15 and likely not an M1. Even a .50 caliber bullet armor piercing round breaks up within 3 feet underwater. So, the myth is confirmed regarding being safe at an angle within 3 to 5 feet deep  of water from most projectiles.  But not explosives or explosive rounds because they weren't tested.