This cartridge is nearly a straight case with a .500 diameter, that gages out on the mouth as does the 9MM, & 45 ACP, etc.. The rebated rim is cut down to .444 diameter used by the 7.62 X 39 to allow the use of the standard AR15 bolt. Its size and length allows the use of the AR15 magazines as single stack. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.