How a Carl Gustav M45 works

The Kulsprutepistol m/45 (Kpist m/45), also known as the Carl Gustav M/45 and the Swedish K SMG, is a 9×19mm Swedish submachine gun designed by Gunnar Johnsson, adopted in 1945 (hence the m/45 designation), and manufactured at the Carl Gustafs Stads Gevärsfaktori in Eskilstuna, Sweden. The m/45 was the standard submachine gun of the Swedish Army from 1945 to 1965. It was gradually replaced in Swedish service by updated Ak 4 battle rifles and Ak 5 assault rifles. The last official user of the m/45, the Swedish Home Guard (Hemvärnet), retired it from service in 2007.

The m/45 SMG was developed in 1944–45, with a design borrowing from and also improving on many design elements of earlier submachine guns. The sheet metal stamping techniques used in making the German MP 40, the British Sten, and the Soviet PPSh-41 and PPS-43 were studied in detail. Two designs were tested in 1944, one from Carl Gustafs Stads Gevärsfaktori and one from Husqvarna Vapenfabrik AB and the prototype from Carl Gustafs Stads Gevärsfaktori was chosen for further development. The first production version was adopted in 1945 as the Kpist m/45. The Danish Hovea M/49 SMG, although similar in appearance, is not a version derived from the m/45. The Hovea was a development of the failed test contender (fm44) from Husqvarna Vapenfabrik AB.

The standard m/45 is a fully-automatic-only weapon without any option for semi-automatic fire. It weighs 3.3 kg (7.3 lbs.) unloaded, and 4.2 kg (9.25 lbs.) loaded with a 36-round box magazine. It is 808 mm (31.8 in.) long with the stock extended, and 550 mm (21.7 in.) long with the stock folded. The m/45 is an open bolt design with a fixed firing pin. The relatively slow cyclic rate of fire (550–600 rds/min.) and low recoil of the bolt-mechanism actuation (straight blowback) makes it easy to control during full automatic fire. Single shots are also easy to achieve (with very little training) by letting go of the trigger before another round is cycled. The m/45 is fairly accurate up to 200 meters.

Accessories include a special sub-caliber barrel (painted silver) for firing blanks and low-powered gallery ammunition. When firing blanks, a cone shaped blank firing adapter must be attached to the threaded muzzle of the sub-caliber barrel (and secured by a clip) to ensure the mechanism has adequate pressure for its blowback operation. Other accessories include night sights (wartime use only) that attach to the fixed day sights (f: protected post, r: L-type), a brass catcher for collecting spent cartridges (peacetime use only, for reloading and recycling),[2] a quick-detachable (by attached cord) ejection port cover (painted bright red) for guard duty which secures the bolt from accidental firing, and a speedloader (rarely issued due to limited availability) that loads the 36-round magazine in seconds. The m/45 was also issued with a standard cleaning kit containing a threaded cleaning rod, threaded jag and a container for the jag, lubricant and cleaning patches. The standard sling issued was made of leather, attached to the rear left receiver and left barrel-sleeve sling bars.

The 36-round straight detachable staggered row box magazine is wider at the rear than at the front, the extra space allows the 9mm Parabellum cartridges to feed more efficiently in dusty environments and sub-zero temperatures. The trapezium design makes the magazine very reliable, because magazines of parallel-side design are more likely to jam under adverse conditions. The magazine was used post-war by Finland in the m/31 Suomi under the designation m/54, a distinguishing feature of the variation m/55 (made by Lapua) is a steel wire carrying loop mounted at the bottom front edge. The basic design idea of the m/45 magazine was also used for the magazines of the Czech model 23 and model 25 and the French MAS submachine guns.

The m/45 has no safety switch. Instead the m/45 is put in "safe" by sliding the cocking handle into a short side-slot above the main (lock) slot. In the example US Army photograph, this short safety side-slot is visible behind the rear L-sight. This design feature results in a somewhat longer time to ready the weapon for firing, because the soldier must remove his right hand from the pistol grip and trigger, as if operating a bolt-action rifle. When the m/45 is unloaded the bolt is locked in place in the bolt-forward position by pushing the cocking handle inwards, engaging a hole in the lower left receiver wall.