Both MP 38 and MP 40 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 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 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 MP 38s 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 MP 38 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 MP 40.
One idiosyncratic and visible feature on most MP 38 and MP 40 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 MP 40 was generally reliable, a major weak point was its 32-round magazine. Unlike the Thompson's double-column, dual-feed magazine, the MP 38 and MP 40 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.