The MP 40 submachine guns are
open-bolt,
blowback-operated
automatic arms. The only mode of fire is automatic, but the relatively low rate of fire permits 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 serves as a safety by pushing the head of the handle into one of two separate notches above the main opening; this action locks the bolt in either 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. The MP 38 also features longitudinal grooving on the receiver and bolt, as well as a circular opening on the magazine housing. These features were eliminated on the MP 40. of the German Army with an MP 38 in 1943|alt= One feature found on most MP 38 and MP 40 submachine guns is an aluminum, steel, or Margolit (a variation of
Bakelite) resting bar under the barrel. This was used to steady the weapon when firing over the side of open-top
armored personnel carriers such as the
Sd.Kfz. 251 half-track. A handguard, also made of Margolit, is located between the magazine housing and the Margolit 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 has 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 found on the
Thompson M1921/1928 variants, the MP 40 uses a double-column, single-feed version. 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 handguard on the underside of the weapon or the magazine housing with the supporting hand to avoid feed malfunctions. ==Usage==