The MP 40, developed in 1940, was a relatively simplistic blowback-operated fully automatic-only submachine gun, and was a refinement on the MP 38. One of the better features of the MP 40 was a mechanism that reduced the rate of fire, which helped the operator maintain control of the weapon when firing. The only severe drawback to the MP 40 was the lack of a hand guard on the front, which could result in the operator burning his hand.
MP-40 on Wikipedia