High Quality Content by WIKIPEDIA articles! The 6mm Lee Navy aka 6mm U.S.N. aka 236 Navy is an obsolete American rifle cartridge. It was the service cartridge of the United States Navy and Marine Corps from 1895, officially replacing the .45-70 Government round, and was the first small-bore high-velocity smokeless powder cartridge to be adopted by either service. The 6mm Lee Navy was in turn replaced by the .30 Army (.30-40 Krag) cartridge in 1899.