Saw a posting over on CSP website where a fellow claims he has a 12 year old COLT barrel that is factory marked 5.56 NATO and it really has  a .223 chamber. He claims to have checked it with both 5.56 headspace gauges and .223 headspace gauges. Anyone have similar barrel or heard of the same situation?

What a dork.  You can't even check a .223 chamber on an AR properly with a .223 gauge.

The 5.56 case has a slightly longer neck length, as such the .223 can be chambered and fired in a 5.56 weapon.
If you try to fire 5.56 out of a .223 chambered weapon, you can have an over pressure problem resulting in a KB due to the longer neck of the 5.56.

Actually, the necks of the cases are the same.  The 5.56 chamber has more leade, or freebore, to allow out of spec or damaged ammo easier feeding.

The higher pressure associated with firing 5.56 ammo is because it is loaded to military specifications, not SAAMI specs.

The AR15, whether chambered for .223 or 5.56, is quite strong, and can well shoot either ammo in either chambering, and this has been done for years by many people with no ill results, notwithstanding Bushmaster's warning (which is inconsistent with Armalite's advise, and others).  I feel that Bushmaster touts this to promote their "5.56" chamber over that of other manufactuers.

SAAMI's warning does not specifically deal with use of 5.56 ammo in .223 chamberd AR15's.  It deals with use of 5.56 ammo in the very weakest of .223 sporting arms that may be in use.  And ALL of the AR15 rifle and barrel makers know that it is possible, no, very likely, that 5.56 ammo will be fired in their .223 barrels.  If it were really a safety issue, all of them would make only "5.56" chambers.

SEMI-AUTO chambers are generally looser than bolt actions.  This is necessary to allow smooth feeding and extraction.  It is also generally advised in reloading that ammo for semiautos be full length resized, rather than just neck sized as is common by target/varmint shooters who reload.

This looser chamber is why headspace gauges may give erronius indications in ALL AR15's, whether .223 or 5.56 chambers.  It is a semi auto, it is supposed to be looser.

Yea, what he said.

It's all here; [url]ammo-oracle.com[/url]

I am aware of the differences between .223 SAAMI and 5.56 chamber, My question was if anyone has a COLT made AR15 barrel that is marked 5.56mm NATO and is really a .223 chamber, not 5.56mm.
The fellow claims he has a .223 chamber in a COLT marked 5.56mm barrel.
comments anyone??

I explained why he could not tell by using headspace gauges.  He is BSing you.  Both chamberings headspace the same in an AR15.  And Semiauto's headspace larger than boltguns.

He has no idea what he has.

[url]www.armalite.com[/url]

On the left, click Tech Notes, and read #45.