AR 15 barrel caliber identification help. (update)

[Daddymac]

I have a 20" AR barrel and the only markings on it are MOS 1-7 HC MP I have done a thorough search on the barrel and there is no caliber marking anywhere.

I think this is a barrel I removed from a bushmaster carry handle when I replaced the upper with a flat top and 16" barrel, but I am not 100% sure. The bush master lower is marked .223/5.56 but again I am not 100% sure the barrel came off of this rifle.

How can I be sure the barrel is good for 5.56? I know the safe play is to just shoot .223 but I would like to know.

I have done some digging on other sites to no avail, any help would be appreciated.

Thanks.

 

[Park ranger]

Jam a bullet such as an 80 grain SMK in a known 5.56 or W and do it again in a 223 marked gun? Compare to the barrel in question???? Just a guess.

 

[AccelR8]

Chamber cast, Roto Metals has a chamber casting alloy and IIRC they have instructions on their site. If not, there are plenty of instructions for chamber casting out there on the net. Cerrosafe is another alloy which can be used.

Measure your chamber cast carefully with a digital caliper or dial caliper if you prefer) and compare the critical dimensions (feebore/leade) with diagrams on the net.

Even so, it might be a tough call!

 

[ebb]

The 1-7 tells me it was made in this century and 223 and 5.56 have evolved into one.

 

[RegionRat]

If this is an MOS bbl it is 4150. Hard Chrome lined, Mag Particle inspected.
And that would make it 5.56 NATO.

Are you sure it is 20" or is it 16"?

 

[Fast14riot]

Even if wilde chamber, that was originally a match chamber for 223. If a LC round chambers and fires with enough neck clearance i wouldn't worry about it. If you reload, who cares. If you're going to sell it, state that in the ad.

 

[Daddymac]

If this is an MOS bbl it is 4150. Hard Chrome lined, Mag Particle inspected.
And that would make it 5.56 NATO.

Are you sure it is 20" or is it 16"?

Thanks, it's 20" for sure, I guess headspace gauges will be no help, I do have .223 gauges the BCG will close on the GO and will not close on the NO GO. This does not surprise me even though the 5.56 "can" have a more generous headspace that does not mean that it will, I think the difference of concern is in the leade.

I am going to use my Hornady OAL gauge and compare one of my AR's with a known .223 chamber with this barrel to see if there is a noticeable difference to the lands.

 

[RegionRat]

I am going to use my Hornady OAL gauge and compare one of my AR's with a known .223 chamber with this barrel to see if there is a noticeable difference to the lands.

Yup, no need to take risks, you will want to inspect it as well as you can regardless. It is easy enough to stay out of trouble.

 

[butchlambert]

Thanks, it's 20" for sure, I guess headspace gauges will be no help, I do have .223 gauges the BCG will close on the GO and will not close on the NO GO. This does not surprise me even though the 5.56 "can" have a more generous headspace that does not mean that it will, I think the difference of concern is in the leade.

I am going to use my Hornady OAL gauge and compare one of my AR's with a known .223 chamber with this barrel to see if there is a noticeable difference to the lands.

Different headspace?? Man, I get tired of the 5.56 and 223 crap!

 

[Daddymac]

Different headspace?? Man, I get tired of the 5.56 and 223 crap!

I fully realize the cases are exactly the same on the outside so one would think that the chambers would be the same, but this is a cut and paste from the Forster Products website explaining their NATO headspace gauges.

Though commonly thought to be identical, the 5.56 NATO allows a slightly longer headspace than the commercial .223 caliber.

 

[Daddymac]

I have found an article at Ultimate Reloader that gives a very good explanation, sorry I can't post a link but you can search this on their site if interested.

223 -vs- 5.56: FACTS and MYTHS​

It is really worth the few minutes to find and read.

 

[gunsandgunsmithing]

I fully realize the cases are exactly the same on the outside so one would think that the chambers would be the same, but this is a cut and paste from the Forster Products website explaining their NATO headspace gauges.

Though commonly thought to be identical, the 5.56 NATO allows a slightly longer headspace than the commercial .223 caliber.

Only MAX allowable hs is different. Go gage is same.

 

[VA_XTC_Shooter]

Only MAX allowable hs is different. Go gage is same.

223 GO is 1.4363, the GO called out in the TDP for 5.56 NATO is 1.4646.

0.001 different. According to COLT, the extra 0.001 is to allow for reliability. As the gun is fired rapid fire the chamber gets hot and HS of the chamber can shrink. (This is according to COLT in their armorers classes). I can't find anything to back that up though.

I would buy that the extra 0.001 is for reliability in the sense you may get some dirt on the bolt face and still function, but I don't buy the COLT explanation.

The 0.001 HS has nothing to do with the pressure arguments between 5.56 and 223. That all comes from the longer throat and larger diameter freebore of the 556 chamber.

 

[gunsandgunsmithing]

223 GO is 1.4363, the GO called out in the TDP for 5.56 NATO is 1.4646.

0.001 different. According to COLT, the extra 0.001 is to allow for reliability. As the gun is fired rapid fire the chamber gets hot and HS of the chamber can shrink. (This is according to COLT in their armorers classes). I can't find anything to back that up though.

I would buy that the extra 0.001 is for reliability in the sense you may get some dirt on the bolt face and still function, but I don't buy the COLT explanation.

The 0.001 HS has nothing to do with the pressure arguments between 5.56 and 223. That all comes from the longer throat and larger diameter freebore of the 556 chamber.

What is TDP? I can see if a Mfg goes a thou longer by choice and still be within applicable specs. Frankly, I'd say it was more marketing by Colt than anything of real value. I'd look at cartridge vs chamber drawings too. Bottom line, with go and no go of the tighter of the two, to me, is still no change. No different than stated tolerances on a blueprint. JMHO. No wonder there's so much confusion but can we agree that a bbl chambered to a depth to datum of 1.4636-1.4666 and they are interchangeable, dimensionally. Those dimensions seem to fall within any print I've seen for either.

 

[M-61]

I had two in my life. Couldn’t get rid of them fast enough. One was issued to me the other I purchased.
I always thought the difference between barrel markings was due to the two different leade angles. Nothing else.

 

[ebb]

Mr Wick a member here cracked the code on this his first week. Made us all look foolish. He is the go to guy on all things 5.56 V 223.

 

[M-61]

Mr Wick a member here cracked the code on this his first week. Made us all look foolish. He is the go to guy on all things 5.56 V 223.

I missed his post. What #?

 

[VA_XTC_Shooter]

What is TDP? I can see if a Mfg goes a thou longer by choice and still be within applicable specs. Frankly, I'd say it was more marketing by Colt than anything of real value. I'd look at cartridge vs chamber drawings too. Bottom line, with go and no go of the tighter of the two, to me, is still no change. No different than stated tolerances on a blueprint. JMHO. No wonder there's so much confusion but can we agree that a bbl chambered to a depth to datum of 1.4636-1.4666 and they are interchangeable, dimensionally. Those dimensions seem to fall within any print I've seen for either.

TDP is technical data package, It is the prints for all parts of the COLT M16M14. This is where the 0.001" longer HS came from compared to commercial 223.
Again, I am not buying the HS difference is that concerning, just sharing the why a lot of people say so.

 

[Daddymac]

Ok, lets call it .223
Went out today and test fired the rifle with some 5.56 and blew a primer, so....

 

[gunsandgunsmithing]

Ok, lets call it .223
Went out today and test fired the rifle with some 5.56 and blew a primer, so....

Sooooo, what? Blown primers are virtually always pressure, not headspace. Pierced primers, seldom due to pressure but pressure is a contributing factor. So, what did you have and what are you saying?