I have an Olympic Arms upper (11.5 inch)obtained in the mid 90s and I was wondering if this has the military or the civilian chamber dimensions. I have shot both military and civilian brands of ammo through it with no issues at all but with all this talk of the differences between both and possible consequences of shooting military ammo through a civilian chamber I am now curious as to what the dimensions actually are. There are no markings on the barrel or receiver to indicate any specific caliber. I believe the twist may be 1 in 9.

Also, would the Armalite 14.5 inch uppers be civilian chamber dimensions or are they military??

Its not whether its civilian vs milspec, its .223 Remington vs 5.56 NATO or other variations.

Most likely its a 5.56 chamber but I would call and ask to be sure. The Oly I use to own had a 5.56 chamber and was marked under the handguards near the barrel nut.

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.

My old (180 prefix) Mini-14 is stamped 5.56.

The Mini's except the Target Mini is a 5.56 chamber. The Target Mini is.223 only. kwg

My OLY Arms upper barrel was marked 5.56 but would NOT extract fired rounds. Took it to gunsmith who insisted there was nothing wrong with the extractor. It shot .223 fine.

I have a couple of Oly barrels, one is a Stainless "Ultra Match", and the other is a plain-Jane 20 in. I recently sold off a carbine with an Oly barrel too. All are/were chambered in .223 commercial, or perhaps I should say were throated in .223 commercial.

Mini 14s had long throats, and Bill Ruger hisself said they could shoot 5.56 spec ammo. This would make sense because local Governments (read prisons) seemed to buy most of them in the old days and used military spec ammo.