how deep should the SB be for keeping pistol shrimp?

I have the same tank and have a Japanese Pistol Shrimp in it. I have about 60lbs of live rock in the tank and the shrimp has lived within the rocks for well over a year now. It has never lived in the sand and has never bonded with any of my gobies. I even bought the goby he was living with at the LFS when I bought it but they separated as soon as I put them in the tank. Goby didn't last more than a few months for some reason but I have 3 other gobies on the tank that have been alive and well for over 3 years. My sand bed is 3-5" deep (depending on the mounds my engineer goby makes), but again, the pistol shrimp has never made it to the bottom on the tank. Always lives in the rocks and I hear him snapping at least 6 times a day so I know he is alive.

Actually I just saw him come out today for the first time in months. He has lived peacefully (besides the snapping) with all the others in the tank, even my cleaner shrimp, for a while now. I know they can be dangerous to others in the tank, especially cleaner shrimp since they eat them, but I have either been very lucky or my pistol shrimp gets enough to eat that it doesn't feel the need to kill anything else in the tank. Not sure if all the snapping is due to him simply stretching his claw or trying to scare away others close by but so far no casualties.

I don't think I would consider a pistol shrimp a "cleaner" though.