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.