How to Build the Suomi M-31

The Suomi M31 was a Finnish submachine gun adopted by the Finnish army and the Swedish army in the 1930s. It played a major role in the defense of Finland against the Soviet Union in the Winter War of 1941. The M31 was a blowback-operated, selective-fire machine gun with a complex "advanced primer ignition" system. The primer of the round being chambered was ignited while the bolt was still in motion. The Suomi M31 fires 9 mm-by-19 mm Luger rounds, fed from a drum or box magazine. The barrel and jacket were of quick-change design for fast replacement in the heat of fire fights. Production of the Suomi M31 was discontinued in 1944, but it was used by the Scandinavian military into the1990s. Rebuilding the Suomi M31 -- following routine field-stripping -- is a straightforward procedure.

Instructions

    • 1

      Install the trigger assembly in the stock. Secure it with a single assembly-screw.

    • 2

      Place the barrel in the receiver. Slip the barrel shroud over the shroud-locking lugs; twist the lugs counterclockwise to lock them in place. Slide the barrel-shroud lock into position.

    • 3

      Reassemble the two sections of the bolt. Reassemble the receiver end-cap and spring, and place them in the receiver.

    • 4

      Pull the charging handle to the rear. Line up the slot in the cap with the extruded portion of the charging handle.

    • 5

      Point the receiver end-cap away from you. Compress the spring by pushing down with your thumb, and screw the cap down on the receiver threads. Back off slightly once the cap is tight. The cap aligns with the slot in the threads.

    • 6

      Release the operating handle. Engage the safety in the trigger guard by pulling it into the rear "Safe" position. Test operability of the gun. Disengage the safety by moving it forward into the "Fire" position. Pull the trigger to dry-fire the weapon; verify that you hear the click of the firing pin.