All of the M1S barrels Ive ever seen were marked 223 if 223 and if not marked were 5.56, so Id say 99% sure its 5.56. If you want to be 100% sure you can take a case and cut the neck with a dremel long ways so you can put a bullet in it by hand tight, then chamber it gently. extract gently and then measure, then do the same in a standard 223 chamber and compare.