My newly purchased M-4 COLT has 5.56MM on the box, and the serial number. The same COLT rifle has .223 stamped on the rifle and the same serial number.
I have fired BOTH cartridges in a RUGER Mini-14 and other rifles with no problems.
The Mini 14 is chambered for 5.56 which means you can use .223 also.If it was chambered in. 223 it would be a no bo to fire a 5.56 in it.