 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 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 , used to hold the bolt in forward position . 
