The Maschinenpistole 40 ("Machine pistol 40") descended from its predecessor the MP 38, 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 weapons designer . 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. The only mode of fire was fully automatic, but the relatively low rate of fire enabled 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 MP 38s, but on late production MP 38s and MP 40s, the bolt handle was made as a separate part. It also served as a safety by pushing the head of the handle into one of two separate notches above the main opening; this action locked the bolt either in the cocked (rear) or uncocked (forward) position. The absence of this feature on early MP 38s resulted in field expedients such as leather harnesses with a small loop that were used to hold the bolt in the forward position.
The MP 38 receiver was made of machined steel, but this was a time-consuming and expensive process. To save time and materials, and thus increase production, construction of the MP 40 receiver was simplified by using stamped steel and electro-spot welding as much as possible.
A hand guard, made of a synthetic material derived from bakelite, was located between the magazine housing and the pistol grip. The barrel lacked any form of insulation, which often resulted in burns on the supporting hand if it was incorrectly positioned. The MP 40 also had a forward-folding metal stock, the first for a submachine gun, resulting in a shorter overall weapon when folded. However, this stock design was at times insufficiently durable for hard combat use.
Although the MP 40 was generally reliable, a major weakness was its 32-round magazine. Unlike the double-column, staggered-feed magazine insert found on the Thompson, the MP 40 used a double-column, single-feed insert. The single-feed insert resulted in increased friction against the remaining cartridges moving upwards towards the feed lips, occasionally resulting in feed failures; this problem was exacerbated by the presence of dirt or other debris. Another problem was that the magazine was also sometimes misused as a handhold. This 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 handhold on the underside of the weapon or the magazine housing with the supporting hand to avoid feed malfunctions.
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