The 40 – 3800 was part of an internal Winchester project called “Armor Machine Gun System” dating from 1968 to develop an improved cartridge based on the .50cal Browning. The name of this cartridge denotes the calibre and velocity, and the overall length of the round was close to the 50cal. to minimise modification of the weapon system. Several different AP and AP-Tracer bullets were tested with most of these having a two-piece construction with machined copper alloy bullet with aluminium tip.