Reising was an assistant to firearm inventor John M. Browning. In this role, Reising contributed to the final design of the US .45 Colt M1911 pistol, the most well-known pistol in history. Reising then designed a number of commercial rifles and pistols on his own, when in 1938, he turned his attention to designing a submachine gun as threats of war rapidly grew in Europe.[1]