The Glock 21 MOS (Modular Optic System) adapts the full-size .45 ACP platform for red dot sight mounting. It is the least common of the full-size MOS variants but serves competitive and duty shooters who prefer the .45 ACP cartridge with the capability to run a miniature red dot. The MOS plate system accepts the same adapter family as other MOS-equipped Glocks. Type: Pistol Action: Semi-Automatic Caliber: .45 ACP Capacity: 13 Year introduced: 2015, Austria Weight: 26.28 oz Overall length: 8.22" Status: Active Intended use: Competition, Self-Defense