 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 . 
