Since CMP designated it so
I don't know for sure but I suspect the military used it for small bore rifle competition until someone looked at the 5.56 round and decided that was redundant.
"Jeez, man, what's another word for 'stupid?'"
"Tactical." -Tom Servo
Marlin 795, 795ss, 39A Mountie, CZ-455 American, CZ-452 Scout, CZ-75 Kadet, AOM160 M1 Carbine, USGI M1 Carbine, M1 Garand, Dan Wesson PM7 1911