The MP5 was first introduced by Heckler & Koch in 1966, under the name HK54. This name comes from HK's old numbering system: the "5" designates the model as a submachine gun, while the "4" identifies it as being chambered for 9 × 19 mm ammunition. The current name dates from when it was officially adopted by the West German government for use by its police and border guard as the Maschinenpistole 5 ("Machine pistol 5", or MP5), in mid-1966. The GSG 9 (the counter-terrorist unit of the German Federal Police) then introduced the MP5 to other Western counter-terrorist units. (less info)