Years ago, my late uncle, was a cab driver in Chicago.
A guy with what appeared to be a firearm, attempted to rob him. He wrestled with the guy over the back of the seat, trying to take the "gun" away, during which time, the cab went up on the sidewalk and through the wall of a drycleaners.
After he regained his senses, the robber bolted from the wrecked cab. My uncle grabbed the High Standard .22 derringer he kept in the cab. He fired a shot at the robber, whom he believed to still be armed. He missed, and in the process, shot himself in the index finger, breaking it. He had pulled the trigger with his second finger, with his index finger in front of the muzzle. Returning to the cab, he found a water pistol on the floor of the passenger compartment.
I don't know if the wouldbe robber was ever caught. My uncle was not arrested or charged, this being in excess of forty years ago, when the Chicago PD was crooked, but not gratuitously malicious.
Lessons learned:
1. Derringers suck. Reserve them for contact shots.
2. Know your firearm and practice with it, at least SOMETIMES.
3. Being an armed robber is dangerous. Your water pistol can get you shot just as easily as a real gun could, and probably justifiably under the law. Your victim doesn't have to KNOW his life is in danger. He just needs to have a reasonable belief that it is. In most places, a realistic toy gun qualifies.