Mk22:

Learn about the Mk22 tranquilizer pistol in Metal Gear Solid: Portable Ops, obtainable after getting your sneaking suit.

The Mk22 is an automatic pistol developed for special operations use, created in response to demand from U.S. Navy SEALs. It can be equipped with a sound suppressor (a device that reduces noise). The pistol features a slide lock lever, primarily designed for single-shot firing, which enhances the sound-reducing capability of suppressors. It uses 9mm tranquilizer ammunition.

Obtained: You get the Mk22 after you acquire your sneaking suit. Some male doctors carry them, as does the Paramedic. Female scientists also carry them.