What is "twenty-two pistol"

Usage examples of "twenty-two pistol".

Core realized what she intended and turned, pulling out a twenty-two pistol.

I didn't find any hints, but I did find a long-barreled twenty-two pistol, and a tiny thirty-two, tucked away under clothing in a dresser drawer.

He opened the box and from it brought forth a single-shot Derringer twenty-two pistol.