Does the bullet explode if fired under water?

Answer 1

Your question is a bit ambiguous, but let's see what we can do.

First, not many guns will fire effectively under water, and attempting to do so could be dangerous. Since water does not compress, if the barrel fills with water, it could fracture or explode. And, because water is a heavier medium than air, if you were able to fire a gun submerged, the distance of travel for the bullet will be significantly less.

Second, recognizing the possibility that you might be looking for the penetration potential of a bullet fired into water and the result to the round, consider again the density of water. Bullets fired into water do not penetrate well, typically not further than a couple feet. Larger rounds such as .50 cal, .44, .45, and 7.62 mm tend to fracture upon contact, and all rounds dump their ability to penetrate much past a foot or two depth.

Finally, bullets do not "explode." Hollow points and soft tips tend to mushroom. These and smaller caliber rounds can fragment, especially if they hit a solid object. Teflon or ballistic tipped ammunition tend to slip through or tumble. Even crosshatched rounds such as Black Talon bullets don't explode; such are engineered to fracture in an even pattern so to cause dramatic soft tissue damage.