Solidly built with the use of brass for the magazine well, and a bayonet mount, it entered service in 1940. The magazine and the bolt of the MP 28 could be used in the Lanchester. The British Sten used the side-mounted magazine configuration and a simplified version of the open-bolt firing operating system of the MP 28. The OVP 1918, an offspring of Revelli's Villar Perosa 1915, inspired Heinrich Vollmer for his telescopic bolt used in the VPM 1930, EMP, MP 38, MP 40 and MP 41. The MP 18 remained in limited service with the German armed forces during the Second World War, specifically with the Sicherheitsdienst, later eastern foreign divisions of the Waffen SS and also with Kriegsmarine coastal artillery units. The Soviet Union made a similar use of MP 28 design in their PDD-34 sub machine gun in 1934. Further development of the PPD-34 led to the simplified PPD-40 and PPSh-41. Design details The MP 18 was a heavy weapon, weighing over when fully loaded. The receiver tube was very thick (around 3 mm), compared with later World War II submachine guns with half that thickness or less, such as the Sten gun or MP 40. Though Schmeisser designed a conventional 20-round-capacity "box" magazine for the weapon, the testing commission, for practical reasons, insisted that the MP 18 be adapted to use the 32-round TM 08 Luger "snail" drum magazine that was widely used with the long-barreled version of the Luger pistol. Like many other open-bolt designs, the MP 18 was prone to accidental discharge. If the buttstock of a loaded gun was given a hard knock while the bolt was fully forward, the gun could accidentally fire because of the bolt overcoming the action spring resistance and moving rearward enough to pick up a round, chamber it and fire. Soldiers liked to leave the bolt of their firearm in this closed or forward position, so dirt and debris would not enter the barrel and chamber. This "bolt-closure" practice acted as a dust cover for the weapon's chamber, preventing a malfunction from occurring because of the presence of debris, but making accidental discharge more likely. The German police asked for external safeties on their MP 18s, and universal bolt-locking safeties were added on all the submachine guns used by the police. Later submachine gun designs like the Sten and the MP 40 were modified to allow the