Symbolism, Imagery, Allegory
Ear, Eye, and Arm have a special gun that emits vibrations that stimulate the sleep center of the brain. In other words, it puts people to sleep, knocking them out for fifteen minutes before they wake up again. Hey, it sounds much safer than actually shooting someone. In fact, it sounds kind of awesome. We'd love to accidentally shoot ourselves into a fifteen-minute nap from time to time.
Don't go thinking the detectives shoot the thing all the time, though. Quite the opposite, in fact. Listen to what we're told about it:
Hanging on the wall were the only things of value in the whole office: three Nirvana guns, obtained at great expense when the detectives opened their office. They had been fired only once at the police training range. (6.8)
The nirvana gun shows that the detectives don't have much experience when it comes to real violence. Sure, they may talk a big game, but they've never even shot their guns outside of the shooting range. At the same time, though, the guns give the detectives some street cred. Everyone recognizes the nirvana guns when Ear, Eye, or Arm take them out around town, and since not everybody can get their hands on guns like these, they command respect, which is pretty valuable…especially since the detectives don't actually have much experience.