Only way to really tell is a chamber cast.
Most the old Olys (pre-94) I've had were stamped on the barrel directly in front of the front base and usually only " 5.56" and no twist rate.
I have same style/age Oly barrel you describe, and its an HBar stamped 5.56 over S.S. for stainless steel.
Most AR styled rifles are 5.56 chambered, exceptions I can recall being the early Colt SP1s as I believe some were marked .223, but I'd have to research that again. Ruger Mini-14 were .223 chambered, not an AR, but a hi-cap consumer of similar ammo.
