A group of engineers from Heckler & Koch created the Heckler & Koch MP5, a 9x19mm Parabellum submachine gun, in the 1960s. Today, the MP5 is available in more than 100 variations and clones, some of which are semi-automatic. In over 40 countries worldwide, numerous armed forces, law enforcement agencies, intelligence agencies, and security groups have adopted the MP5, making it one of the most frequently used submachine guns in the world. The MP5A2, a lightweight, air-cooled, selective fire delayed blowback powered 9x19mm Parabellum rifle with a roller-delayed bolt, is the primary version of the MP5 series. It fires from the closed bolt position (bolt forward). The MP5 has a hammer firing system. The trigger group is located inside a replaceable polymer trigger module that also functions as the manual safety toggle. It has a three-position fire mode selector and an integrated pistol grip. In addition to 9x19mm Parabellum (used in the MP5A1, A2, A3, A4, A5 versions), the MP5 models also employ other calibers: 10mm Auto (MP5/10), .40 S&W (MP5/40), and .22 LR (the rifle and pistol variants).