5.56 chamber

[aintright]

How would you go about determining if a rifle is 5.56 chamber or 223 ? A friend has two older rifles , maybe twenty five -thirty years old that have C MP CHROME BORE and the other is C MP C but no markings as to the chamber . Thanks , Kenneth

[dave]

chamber cast?

[aintright]

Ok , thanks , Kenneth

[BigMo]

Those sound like US Military A1 barrels.
The markings are Colt and passed their inspection.
Unless you have reason to think they were some how rechambered (unlikely due to the chrome lining) they are 5.56.
That and I think it is physically impossible (due to the dimensions involved) to take a 5.56 and rechamber it to 223.

[da gimp]

Big, I am inclined to say that those markings are on Colt's civilian AR's too & they are all .223 Rem to the best of my knowledge.... it just might be that both types have similar marks for proofing..........

[aintright]

Thanks Big Mo , I considered the chrome lining as probally mil spec also , but wanted to make sure . I was playing with some AR's today that are stamped 5.56 as I recently read that the 5.56 ammo would shoot somewhat better in the 5.56 chamber . One of the AR's was scoped and I could see a difference in groups with the 5.56 ammo grouping better .
Thank you too Gimp , did they make commercial rifles with chrome bores ?
We have heard all the nay sayers to shooting 5.56 in 223 chambers is why we wanted to know , but you probally already figured that . One of his lowers are DPMS And the other is stamped E.A. , I assume it is Eagle Arms . The E.A. Is in a Car-15 configuration . Thanks , Kenneth

[Tuna]

The difference between the 5.56 and the .223 is the amount of distance between the chamber mouth and the rifling in the barrel. It's longer on a military spec 5.56 barrel then a commercial rifle. The Colt AR 15 is made to the 5.56 spec even if stamped .223 and Colt commercial barrels are chrome lined. They are the same as their military barrels. There is no difference between them other then what is stamped on them.

[BigMo]

By all mains If in doubt- chamber cast but that is my 2$................

Da Gimp:
I'll respectfully disagree.
The issue and confusion for Colt made AR's is that some (especially older) receivers were marked 223. But the barrels were their Military barrels and so 5.56mm.
They took military produced and marked barrels for their early commercial 223 rifles. Of course for the other calibres, they had commercial barrels ie 9mm etc. And recently with the 1/9 twist barrels- those are clearly marked as such (and 5.56 chambers)- those 1/9 are not military.

But the OP clearly describes Colt (because they are marked that way) A1 barrels. So they are 5.56mm chambers.

"The Black Rifle II" by Bartocci breaks all the parts done extremely well and explains what I just did regarding the barrels. See page 285.

It's a common confusion about Colt AR's- the receiver on some is marked 223 but the barrel (if original Colt) is 5.56..........

[aintright]

Thanks to all . Looks to be another good day for test firing some guns . Kenneth

[budster]

Correct me if I am wrong but, I remember that C=chrome bbl. and MP is Magnafluxed Particle checked and the BBL. Maker was a sub-contractor of Colt.

[BigMo]

Budster:
According to Black Rifle II----
-the very first 18,000 barrels made were made by Winchester for Colt. NO markings at all and 1/14" twist but still 5.56 chamber. After that it was solely Colt made barrels until the other contractors began producing M16's.

-Summer of 1963- Colt was still the only AR producer. Colt switched the barrels (5.56) to 1/12 and so had a "12" on the bottom of the bl. Still 5.56. But soon after started marking the barrels also with "CC" for chrome chamber and "CB" for chrome bore. So they would have "12" and some had CC and/or CB. There was an "M" under the front sight to indicate the MP testing and a small triangle Colt inspection mark as well. At some point- the "M" moved to the rest of the info on the front of the barrel and the triangle went away.

-1967- The chrome plating became standard. Also other companies had begun producing barrels (all 5.56mm) and so you see what for example the OP has:
Colt Magnetic P article (tested) Chrome Bore and the other barrel --same thing but the second "C" indicating chrome chamber.
For example a "SAK" produced barrel might read---SAK MP Chrome Bore.

-But all these barrels had a 5.56 Chamber as that was the standard