A tranquilizer gun (or tranq gun) is a handheld firearm whose ammunition is non-lethal, and used to knock enemies unconscious. Tranquilizer guns in the Metal Gear Solid games include:

Behind the scenes

In Portable Ops and Portable Ops Plus, the ammunition containers for the Mk22 and Mosin Nagant (9mm T and 7.62mm T, respectively) display a star and crescent, presumably to symbolize nighttime.

In Metal Gear Solid: HD Collection, hitting 100 soldiers with tranquilizers in Metal Gear Solid 2 will result in the unlocking of the achievement/trophy "Don't Taze Me, Bro."

On November 8, 2016, an update was issued that allowed for the development of the Anti-Tranq Agent. It is available under the security devices tab. It worked from grades 4 to 6.

"Anti-tranquilization agent.
Stops tranquilizers up to a certain grade from circulating through the body over time.
―Anti-Tranq Agent description in Metal Gear Solid V: The Phantom Pain