How much water does a bullet have to pass through before it's harmless?

It does indeed depend on the caliper of the gun, but I think the most depth penetration the Mythbusters got was around 3 feet with low caliper, and even less with the higher caliper. This was because at higher velocities, the bullets actually broke up quicker with high caliper guns then with lower caliper. By the way, the myth they were trying was saving your live from gun fire by diving under the water.

wow, fun question

I am already picturing the viscosity equations

after looking at the other answers it appears 8 ft

hmm

i think the most important influence a body of water would have on any projectile is the unpredictable change in path/direction

bullet/shotgun pellets are spinning through the air. the change is medium (air to water) and the impact angle will greatly distort/change the desired path of the projectile

PLEEZ DO NOT TRY DEMONSTRATING THIS with people and a swimming pool! That would be extremely dangerous!

Mythbusters did a whole segment on this. Try the Discovery Channel website and look up the show. It's not a few inches. If I remember correctly, about 5 - 8 feet was needed, depending on the gun.

all depends on the bullet and the caliber of it.

mythbusters did a show about this and found that some bullets don't have much impact at all when shot into water.

don't believe everything you see in movies. they often stretch reality to make the story work and figure that if it seems plausible, then the average spectator won't question it's validity.

kind of like one sword cutting another sword in half....the proved this movie favorite as not possible...

take care.

Depends on the velocity and bullet mass. It does not take much, actually.

Jeffery S, is exactlly right 3' or less.