H&K UMP 40 Sub-machine gun action

Me shooting a HK UMP. There are three versions of the UMP: the UMP45, firing a .45 ACP cartridge; the UMP40, firing a .40 S&W cartridge; and the UMP9, firing a 9x19mm Parabellum cartridge. Apart from the different chambering, all versions feature the same basic design, the most noticeable difference being the curved magazine used on the UMP9. All three versions of the weapon can be converted to any of the available chamberings via replacement of the bolt, barrel, and magazine.