Link: http://www.straightdope.com/mailbag/mgununderwater.htm (Nice report, btw, Bricker!)
So, guns designed to be used underwater are better than their normal counterparts in water. How about the reverse? Do guns designed for a water environment suffer the same drop-off in effectiveness after only a few feet, or are they (comparitively) better in air than normal guns are in the water? If someone’s waving an H&K P11 at me in the middle of the Sahara, should I be worried?
At the very least, they won’t be accurate at a distance, because they don’t have rifling.
John W Kennedy is on the right track. Rifling, the spiral grooves inside a barrel that impart a spin to the bullet as it’s fired, have no particular use underwater; there is no advantage gained by spinning a projectile underwater. To the contrary, it would create drag and, depending on the speed of the spin, even low pressure cavitation bubbles, neither of which would be helpful in either stability or distance for an underwater projectile.
On the other hand, not spinning a projectile that’s fired in air is counter-productive. A spinning projectile benefits from gyroscopic precession, and without it the projectile will begin to yaw and then tumble.
Now, don’t get me wrong – a tumbling projectile is still very dangerous, and can cause significant body damage if it hits. The problem is that accuracy over any distance will be compromised. I’d say that comparatively, the dangers of an H&K P11 in the Sahara are comparatively greater than a Ruger P90 underwater, simply because the water blunts penetration as opposed to accuracy. It’s fair to say, though, that neither “fish out of water” scenario is ideal.