The procedure required to install the M1C-type mounts through drilling/tapping the hardened receiver reduced accuracy by warping the receiver. Improved methods to avoid reduction of accuracy were inefficient in terms of tooling and time. This resulted in the development of the M1D, which utilized a simpler, single-ring Springfield Armory mount attached to the barrel rather than the receiver. The M1C was first widely used during the Korean War. Korean War production was 4,796 M1Cs and 21,380 M1Ds; although few M1Ds were completed in time to see combat.