HK MP5

The Heckler & Koch MP5 (from German: Maschinenpistole 5, "machine pistol model 5") is a 9mm submachine gun of German design, developed in the 1960s by a team of engineers from the German small arms manufacturer Heckler & Koch GmbH (H&K) of Oberndorf am Neckar.
The MP5 is currently one of the most widely used submachine guns in the world,^[1] having been adopted by numerous military, law enforcement, intelligence and security organizations. In the 1990s, Heckler & Koch developed the Heckler & Koch UMP, the MP5's successor, though both remain in production.^[2]

The primary version of the MP5 family is the MP5A2, which is a lightweight, air-cooled, selective fire delayed blowback operated 9x19mm Parabellum weapon with a roller-delayed bolt. It fires from a closed bolt (bolt forward) position.
The fixed, free floating, cold hammer-forged barrel has 6 right-hand grooves with a 1 in 250 mm (1:10 in) rifling twist rate and is pressed and pinned into the receiver.
(Sources from Wikipedia)