The MP40 (Maschinenpistole 40, literally "Machine Pistol 40") is a submachine gun developed in Germany and used extensively by paratroopers and platoon and squad leaders, and other troops during World War II. The MP40 was characterized by its relatively low rate of fire and low recoil.
Other changes resulted from experiences with the several thousand MP38s in service since 1939, used during the invasion of Poland. The changes were incorporated into an intermediate version, the MP38/40, and then used in the initial MP40 production version. Just over 1 million would be made of all versions in the course of the war.
The MP40 was often called the "Schmeisser" by the Allies, after weapons designer Hugo Schmeisser. Hugo Schmeisser himself did not design the MP40 but held a patent on the magazine. He designed the MP41, which was a MP40 with a wooden rifle stock and a selector, identical to those found on the earlier MP28 submachine gun. The MP41 was not introduced as a service weapon with the German Army, but saw limited use with some SS and police units. They were also exported to Germany's ally, Romania. The MP41's production run was brief, as Erma filed a successful patent infringement lawsuit against Schmeisser's employer, Haenel.
Both MP38 and MP40 submachine guns are open-bolt, blowback-operated automatic arms. Fully automatic fire was the only setting, but the relatively low rate of fire allowed for single shots with controlled trigger pulls. The bolt features a telescoping return spring guide which serves as a pneumatic recoil buffer. The cocking handle was permanently attached to the bolt on early MP38s, but on late production MP38s and MP40s, the bolt handle was made as a separate part. It also served as a safety by pushing the head of handle into a separate notch above the main opening, which locked the bolt either in the cocked or forward position. The absence of this feature on early MP38s resulted in field expedients such as leather harnesses with a small loop, used to hold the bolt in forward position.
The receiver was originally machined steel but this was a time-consuming and expensive process. This prompted the development of a simpler version that used stamped steel and electro-spot welding as much as possible. The MP38 also features longitudinal grooving on the receiver and bolt, as well as a circular opening on the magazine housing. These features were suppressed on the M38/40 and MP40.
One idiosyncratic and visible feature on most MP38 and MP40 submachine guns was an aluminum, steel, or bakelite resting bar or support under the barrel which was used to steady the weapon when firing over the side of open top armored personnel carriers such as the Sdkfz 251 half-track. A handguard was located between the magazine housing and pistol grip and was made of synthetic material derived from bakelite. The barrel lacked any form of insulation, which often resulted in burns for the supporting hand if it strayed. It also had a compact folding metal stock, the first for a submachine gun, resulting in a shorter weapon when folded, but it was at times insufficiently durable for hard use in combat.
Although the MP40 was generally reliable, a major weak point was its 32-round magazine. Unlike the Thompson's double-column, dual-feed magazine, the MP38 and MP40 used a double-column, single-feed design. The single-feed resulted in increased friction against the remaining cartridges moving upwards towards the feed lips, occasionally resulting in a failure to feed; the problem was exacerbated by the presence of dirt or dust. Another problem was that the magazine was also sometimes misused as a handhold, which could cause the weapon to malfunction when hand pressure on the magazine body caused the magazine lips to move out of the line of feed, since the magazine well did not keep the magazine firmly locked. German soldiers were trained to grasp either the intended handhold on the underside of the weapon or the magazine housing with the supporting hand to avoid feed malfunctions.
Unlike the impression given by popular culture, MP40s were generally issued only to paratroopers and platoon and squad leaders; the majority of soldiers carried Karabiner 98k rifles. However, experience with Soviet tactics where entire units armed with submachine guns outgunned their German counterparts in short range urban combat caused a shift in tactics, and by the end of the war it was being issued to entire assault platoons on a limited basis.
There were never enough MP40s because raw material and labor costs made it expensive to produce alongside the Kar98 rifles. Due to this, starting in 1943, the German army moved to replace both the Kar-98 rifle and MP-40 with the new MP-43/44 assault rifle, also known in its production model as the StG44.