The MP 40 (Maschinenpistole 40) is a submachine gun chambered for the 9×19mm Parabellum cartridge. It was developed in Nazi Germany and used extensively by the Axis powers during World War II.
Designed in 1938 by Heinrich Vollmer with inspiration from its predecessor the MP 38, it was heavily used by infantrymen (particularly platoon and squad leaders), and by paratroopers, on the Eastern and Western Fronts as well as armoured fighting vehicle crews. Its advanced and modern features made it a favorite among soldiers and popular in countries from various parts of the world after the war. It was often called "Schmeisser" by the Allies, after Hugo Schmeisser, who designed the MP 18, although he was not involved in the design or production of the MP 40. The weapon's other variants included the MP 40/I and the MP 41. From 1940 to 1945, an estimated 1.1 million were produced by Erma Werke.
The Maschinenpistole 40 ("Machine pistol 40") descended from its predecessor the MP 38, which was in turn based on the MP 36, a prototype made of machined steel. The MP 36 was developed independently by Erma Werke's Berthold Geipel with funding from the German Army. It took design elements from Heinrich Vollmer's VPM 1930 and EMP. Vollmer then worked on Berthold Geipel's MP 36 and in 1938 submitted a prototype to answer a request from the Heereswaffenamt (Army Weapons Office) for a new submachine gun, which was adopted as MP 38. The MP 38 was a simplification of the MP 36, and the MP 40 was a further simplification of the MP 38, with certain cost-saving alterations, most notably in the more extensive use of stamped steel rather than machined parts.
The MP 40 was often called the "Schmeisser" by the Allies, after the weapon designer Hugo Schmeisser. Schmeisser had designed the MP 18, which was the first mass-produced submachine gun. He did not, however, have anything to do with the design or development of the MP 40, although he held a patent on the magazine.
The MP 40 submachine guns are open-bolt, blowback-operated automatic arms.