Was the MG42 gun waterproof

It was waterproof in the sense that getting one wet- or submerging it- did not damage the gun. In the sense of "Can it be fired underwater?"- no. This was a gun that fired from an open bolt- when the trigger was pulled, the bolt, held open, would strip a cartridge from the belt, chamber it, and fire it, and eject the fired case. An open bolt permits water to drain from the barrel more quickly, so a gun that had been submerged could be put back into use shortly after being taken out of the water. HOWEVER- the gun used a roller locked recoil system (with gas assist) to operate. When dampened by being underwater, parts would be slowed by water, and would fail to cycle properly. The 8mm cartridge when fired underwater will travel only a few feet.