How does one distinguish a AR-15 barrel that is chambered for the 5.56 x 45mm NATO cartridge from a AR-15 barrel that is chambered for the .223 REM. cartridge?  Is there a gauge/device that one can use to easily determine which cartridge the barrel is chambered for?

It says it on my lower as well as my barrel.

Quoted: It says it on my lower as well as my barrel.

So I can't take a lower that says 5.56mm and shoot 6.8SPC or 7.62x39 through it?

LMAO... smartass.


most barreles are MARKED for their chamber.  If it say's  7.62x39 DONT shoot 5.56 through it.  Only a SELECT few of the manufactures make the chamber & bore in .223 only, where as the 5.56 NATO has the mjority of manufactures and a little nore pressure behind it.

It should be marked. I think Olympic Arms has tight chambers for 5.56 though

I was wondering the same thing myself because of a "Mystery" Barrel I just bought. I decided not to take a chance, and only shoot .223 Rem. through it for now, but if I could REALLY find out for sure that sure would be a help. Sorry for the Newbie question but I am still new to all of this stuff, I just slapped together a couple of rifles so far.

A HUMBLE thanks.

Except for DPMS, who has marked barrels with .223, then claim that they are 5.56 chambered, it should be marked on the barrel - IF AND ONLY IF the manufacturer chooses to mark the barrel - RRA and others don't mark all their barrels.

I am not aware of a way to measure a chamber, unless you used some type of molding plastic and filled your chamber, let it set and pulled it out.  Sounds like a major PITA to me.

IMO, a BIG +1 for the manufacturers who clearly mark their barrels with ALL the required info on that barrel.

Cerrosafe Chamber Casting Alloy
Cerrosafe alloy is the gunsmithing standard for doing casts of barrel chambers, dovetails and other hard to measure areas. Cerrosafe melts at a low 170 degrees. The casting should be removed as soon as solidified and cool to touch. During this time, the casting shrinks to allow extraction from the cavity. Sixty minutes after solidifying the casting will have expanded to precisely the "as cast" dimension. 1/2-pound ingot.

Quoted: Cerrosafe Chamber Casting Alloy Cerrosafe alloy is the gunsmithing standard for doing casts of barrel chambers, dovetails and other hard to measure areas. Cerrosafe melts at a low 170 degrees. The casting should be removed as soon as solidified and cool to touch. During this time, the casting shrinks to allow extraction from the cavity. Sixty minutes after solidifying the casting will have expanded to precisely the "as cast" dimension. 1/2-pound ingot.

Since the case dimensions-and thus the case part of the chamber-are for all practical purposes identical between .223 Remington and 5.56mm NATO, the part that you're interested in is the throat; that's the part from where the bullet sits through the "leade" (where the rifling begins).  The 5.56 leade is longer and more gradual (and the throat itself may be longer for more free initial bullet movement  as well) than in the .223 chamber.

I guess someone with really good eyes could see the difference if they could look at the chamber directly.  The standard for the leade in the two chambers is so different that you can see it; the .223 chamber's leade looks almost like a step from bore to rifling, while the "ramp" effect of the more gradual leade in the 5.56mm chamber is quite pronounced.

you should be able to cast the throat as well.