[Yardstick]

I don’t see how redesigning the receiver solves the problem of drag from the water slowing down the bullet. It seems like the bullet would need to be redesigned, not the rifle.

[Thud]

It's also a question of acceleration inside the barrel. The whole idea of a firearm is that the propellant accelerates the round inside the barrel, with the expanding gases doing all the pushing. If the barrel is filled with water between the round and the tip of the barrel, there will be enormous resistance to the round's acceleration. If the barrel is not filled with water, the round will hit the water so hard exiting the barrel as to greatly compromise accuracy.

I have major issues with this story.