The SIG MKMO (Maschinen Karabiner Militär Oben, Top-ejecting military machine carbine in English) is a submachine gun created by SIG.
The MKMO was a joint collaboration between SIG designers Josef Gaetzi and Gotthard End. Hungarian engineer Pál de Király worked as a consultant on the project; he designed the swiveling magazine housing. The weapon was developed in 1933 and put into production the next year, and it was marketed to military and police forces but failed to turn a profit. Production of the MKMO was very expensive and consequently only 1,228 were made. In 1937, SIG marketed the MKMS, a simplified variant, in an attempt to garner some commercial interest in the design, but it was also met with few sales.
The MKMO used a very ingenious, but overly complicated blowback action that kept the cartridge in the chamber long enough within in the chamber until the pressure in the barrel could decrease to a safe level. This mechanism mirrors the hesitation lock found on the Remington Model 51. This action was made up of a two-part blowback bolt designed by Gotthard End. The MKMO was also the very first weapon to have a folding magazine which could be stored in the stock; this was later mirrored in the Danuvia submachine gun and the Beretta Model 38. This feature allowed the weapon to be transported more easily when the need arose. When the firearm is in battery, the breech block rests slightly forward of the locking shoulder in the frame. When the cartridge fires, the breech block and bolt carrier move together for a short distance rearward, powered by the energy of the cartridge as found in a standard blowback system. As the breech block contacts the locking shoulder, it stops, locking the breech. The bolt carrier will then continue rearward with the momentum it had acquired in the initial phase. This delay allows the chamber pressure to drop to safe levels while the breech is locked and the cartridge slightly extracted. As the bullet leaves the barrel, pressure drops, with the continuing motion of the bolt carrier cams the breech block from its locking recess, continuing the operating cycle.
The MKPO had a shortened barrel, but retained the unique blowback system.
The MKMS has a longer barrel than the MKMO but has a more conventional blowback mechanism.
- The MKMO and its variants had sighting flaws. Both sights were manually adjustable, and could be calibrated from 100 meters to a ridiculous 1000 meters, leading to much confusion in accuracy with many young soldiers who could not operate the sights properly.