I sent an email to Oly, stating that I was advised this barrel is a .223 chamber, and that I would like it to be 5.56.  I asked if I could have it modified or trade it in.  This was the response:

The chamber is in the NATO 556 all of our National Match Barrels are in the
223 Remington thanks
(rep name removed by mn227)
Domestic Sales Rep.
Olympic Arms Inc.
800-228-3471    Order Line
360-459-7940    General Info
360-491-3447    Fax
www.olyarms.com

I thought this was only for current barrels, and older ones were all .223?  I made it clear this was a pre '94 upper.

Any thoughts?  Thanks!

P.S. is this a 1:9?

The only barrel Oly makes in .223 are their S.U.M. barrels, like they said, for match. Everything else is NATO 5.56mm. Oly barrels are stamped 5.56 on the superior aspect of the barrel forward of the sight base. If it is a .223 chamber it will be stamped "S.U.M.".

If you have a 11.5 inch barrel it will have a 5.56 chamber guaranteed unless it is not an Olympic barrel or it was a match barrel that was cut down by the previous owner.

Thanks!

There's a contradictory opinion out there...  I suppose the only way to be sure would be to measure it myself.  

The barrel is marked 5.56

If the barrel is marked 5.56 then that is what it is chambered for.

You have a 5.56mm NATO chamber

Okay dudes, if the XM193 blows up my gun, you'll be the first to know!

LOL

The Oly web site indicates that all 223/556 barrels are marked 556.  The exception is for alternate calibers like 45ACP and 40S&W.  So the 556 marking just indicates the bore diameter and not the chamber dimensions.

New uppers are marked SUM for the 223 chamber, otherwise it is a 556 chamber.

I have been told that at some point in the past Oly changed from all chambers being 223 to just the SUM uppers.  However I have never been able to verify what date that is.

I was told it was 1999.  Maybe I should ask the Oly rep how long he has worked there?

The barrel marking is what counts.

My Colt post ban is marked .223 on the reciever, but is marked 5.56 on the barrel.

Are you sure about this fact? I don't believe I have ever heard of anyone marking a rifle barrel 5.56 that is chambered for .223. Is is possible that the website is stating that all rifles barrels marked 5.56 can shoot 5.56 or .223?

Gentlemen,

Please do not confuse the "5.56" markings on the barrels of all AR barrels to mean that the chanbering is 5.56 NATO. The 5.56 on the barrel in most cases (unless immediately followed by the marking "NATO") is nothing more than a bore dimension.

5.56mm is the bore diameter equivelant of .224", or the same bullet diameter of a 223 Remington, or 22-250. Get my drift?

If your barrel is "30 caliber" all that means is that the bullets is shoots are .308" in diameter. It does not tell you the "chambering" of the barrel. It is a 30 caliber all right, but is it a 30-30, or a .308 Winchester, or a 300 Winchester Magnum or whatever.

Olys barrels have always been marked 5.56, but they were at one time chambering all of their barrels as minimum SAAMI spec 223 Remington chambers. After speaking again with the guys at Oly, they tell me that the switch was made BEFORE the AWB in 1994 (perhaps a year or so..) BUT, with having had som much inventory on the shelf at the time of the ban, that they cannot guarantee that all barrels that left the factory after the AWB were all 5.56 NATO chambers. However, 90% propobably are. Tom @ Oly tells me he will be adding this to the FAQ page on the Oly web site soon.

There you have it. From them to you...

The SN of my lower is between:

T2120-T3758 01/08/1991-04/05/1991

What are my odds this is a 5.56?

(assuming of course a complete rifle)

Either way, Oly needs to get all of their reps on the same page.

Hi MickAr15,

I do not think Colt markings and Oly markings have anything to do with each other. Apples and oranges.

Bob

you wanna know? Do a chamber cast and stop guessing!

Guys older OLY rifles were marked 5.56 but had the .223 chamber.  They didn't switch over till customers started demanding M16 chambers.

Below is the notation from www.MD-AR15.com on Oly chambers (from the [b]AR-15 Brands[/b] page).

Like Zenif says.......have a gunsmith take a cast of the chamber and throat and measure it.  Then you know for sure!