Mine shot my 1" damsel in the face, and it was just fine. Pistols aren't dangerous to fish unless the fish are very small.
If you get snapped by a decent-sized one, it feels like being snapped with a rubber band. Stings a bit, but not bad at all.
I suppose there's a tiny chance that a pistol could pull snails/hermits out of shells, but their usual approach to food is to shoot it repeatedly, snip it a few times with their claws, and then eat it. I'd say that the pistol is just taking advantage of shells laying around.