Modified C.I.A. "Stinger" re-loadable .22 magnum pen-gun is cocked via camera's film advance lever. The magnum round is used to offset the diminished velocity as the bullet penetrates the lens elements. Other models incorporate baffles within a telephoto lens which then works quite effectively as a silencer (supressor).
The C.I.A. Stinger (sold commercially by Military Armaments Corporation as the "Single-Shot Survival Weapon" is perhaps the single most popular weapon the C.I.A.'s Technical Services Division has ever offered. It has seen successful deployment in toothpaste tubes, pens, cigars, etc. It may be swallowed or stowed as a suppository to avoid detection.