What happens is they may drag a snail/hermit down into the burrow, intending to use it for shoring. The critter either crawls out or dies, and the pistol eats it if the latter happens. They aren't hunting on purpose.
Get a tiger pistol shrimp and give him plenty of easy-to-reach bits of rubble and shell, and he should leave them alone. Also, any snail/shrimp nickle sized or over should be fairly safe. The pistol will likely shoot it if it gets too close, but he won't hurt it, he'll just irk it and maybe make it hide.