Gunsmithing Excessive AR Headspace

[DirtyRod]

OK AR gurus ... need your help.

I've been shooting an AR forever recently got into building a few. First was fine but I made the mistake of buying a used barrel for my last. Shoots just fine but I can close the bolt on the NOGO but not on the FIELD gauge. Trying to figure out the plan. I tried a few bolts but they all close on the NOGO.

I've thought about just adjusting my sizing die for the proper headspace the same way I do with the .308 loads for my bolt gun but then I'd have to load separately for both ARs which defeats the purpose of finding a load that they both like. I suppose I could try a headspace setting that is a compromise but that still seems like a guess and I'd rather fix it rather than always wonder if it's OK.

Any other options other than replacing the barrel?

I searched the Semi-Auto and Gunsmithing forums and don't see a whole lot of people actually fixing headspace issues in ARs.

 

[sinister]

Re: Excessive AR Headspace

Closing on the NO-GO tells you the chamber is at the <span style="color: #FF0000"><span style="font-weight: bold">maxim</span>um </span>safe limit.

Closing on a FIELD gage is UNSAFE, as it is past safe dimensions and tolerances.

 

[DirtyRod]

Re: Excessive AR Headspace

Thank you. I went back and re-read the Forster page and it indicates that their Field gauge is slightly LESS than SAAMI max to provide a safety margin so I guess I should be good and will keep shooting it.

Appreciate the response.

 

[TGagnon]

Re: Excessive AR Headspace

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Dirty Rod</div><div class="ubbcode-body">OK AR gurus ... need your help.

I've been shooting an AR forever recently got into building a few. First was fine but I made the mistake of buying a used barrel for my last. Shoots just fine but I can close the bolt on the NOGO but not on the FIELD gauge. Trying to figure out the plan. I tried a few bolts but they all close on the NOGO.

I've thought about just adjusting my sizing die for the proper headspace the same way I do with the .308 loads for my bolt gun but then I'd have to load separately for both ARs which defeats the purpose of finding a load that they both like. I suppose I could try a headspace setting that is a compromise but that still seems like a guess and I'd rather fix it rather than always wonder if it's OK.

Any other options other than replacing the barrel?

I searched the Semi-Auto and Gunsmithing forums and don't see a whole lot of people actually fixing headspace issues in ARs. </div></div>
Reread your post carefully. You say that it closes on the NoGo gauge but not on the field. That isn't possible unless you have a faulty gauge. Second the reason a nogo gauge is called a nogo gauge is that it is not supposed to close on that gauge at all ever. You shouldn't shoot the barrel until you find out if your nogo or your field is faulty and get a replacement or loaner gauge. Then redo your test and ask questions or make judgments from there.
But if it is closing on a nogo and the nogo proves to be gtg then you need to either have the extension turned in one full turn and chamber rereamed or a new barrel.

 

[hdbiker1]

Re: Excessive AR Headspace

As far as I know, the go and no-go gauges are used for new barrel assembly installs. The field gauge is the one to go by with a barrel assembly that has been in service. But I could be wrong.

 

[Sandbogg]

Re: Excessive AR Headspace

Most ars that I have checked close on a no go. The ones that did not usually ended up with live factory rounds stuck in the chamber. A field gauge is bigger than a no go gauge. TGagnon, I think you are confusing a field gauge with a go gauge.

I like my rifle to close on a go gauge, just barely close on a no go and not close on a field. This has worked for me, however YMMV.

 

[Jhnmdahl]

Re: Excessive AR Headspace

Dirty, your understanding is correct. From Forster's instructions:

NO-GO: Corresponds to the maximum headspace Forster recommends for gunsmiths chambering new, bolt action rifles. This is NOT a SAAMI-maximum measurement. If a rifle closes on a NO-GO gage, it may still be within SAAMI specifications or it may have excessive headspace. To determine if there is excessive headspace, the chamber should then be checked with a FIELD gage. The NO-GO gage is a valuable tool for checking a newly-reamed chamber in order to ensure a tight and accurate chamber.

FIELD: Corresponds to the longest safe headspace. If a rifle closes on a FIELD gage, its chamber is dangerously close to, or longer than, SAAMI's specified maximum chamber size. If chamber headspace is excessive, the gun should be taken out of service until it has been inspected and repaired by a competent gunsmith. FIELD gages are slightly shorter than the SAAMI maximum in order to give a small safety margin.

 

[BusterB]

Re: Excessive AR Headspace

Are there any differences in the gauges for .223 vs 5.56 chambers? If so that could account for some of the variations being seen. I'm not sure about any of the chamber specs though.

 

[kennygss]

Re: Excessive AR Headspace

Does the used barrel have a: 223, 5.56 or Wylde chamber? A 5.56 Chamber is sloppy, with a longer throat (usually .226 diameter x .056 long) .223 has a .025 throat, Wylde has a .224 diameter throat with .o61 of freebore. There are other chambers as well, who made the barrel?

 

[DirtyRod]

Re: Excessive AR Headspace

Thanks guys. Appreciate the information and validation. Shoots very well with no pressure signs and my loads are in the midrange.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BusterB</div><div class="ubbcode-body">Are there any differences in the gauges for .223 vs 5.56 chambers? If so that could account for some of the variations being seen. I'm not sure about any of the chamber specs though. </div></div>

The throat and leade are different between the two rounds as BDKen stated but the headspacing should be the same I've read.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: BDKen</div><div class="ubbcode-body">Does the used barrel have a: 223, 5.56 or Wylde chamber? A 5.56 Chamber is sloppy, with a longer throat (usually .226 diameter x .056 long) .223 has a .025 throat, Wylde has a .224 diameter throat with .o61 of freebore. There are other chambers as well, who made the barrel? </div></div>

Colt barrel but here is the twist. It was traded to me as an SBR upper but I didn't realize that it was cut and not a factory SBR when took it in trade. I figured out it was a Colt A1 upper but all the barrel markings were cut off so I'm not sure exactly what it is. I tried to figure out the twist but the closest I could figure was about 1:12 so I'm loading .223 just to be safe. All the Colt literature I've found for the markings on the upper point to .223.

 

[TGagnon]

Re: Excessive AR Headspace

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: jhnmdahl</div><div class="ubbcode-body">Dirty, your understanding is correct. From Forster's instructions:

NO-GO: Corresponds to the maximum headspace Forster recommends for gunsmiths chambering new, bolt action rifles. This is NOT a SAAMI-maximum measurement. If a rifle closes on a NO-GO gage, it may still be within SAAMI specifications or it may have excessive headspace. To determine if there is excessive headspace, the chamber should then be checked with a FIELD gage. The NO-GO gage is a valuable tool for checking a newly-reamed chamber in order to ensure a tight and accurate chamber.

FIELD: Corresponds to the longest safe headspace. If a rifle closes on a FIELD gage, its chamber is dangerously close to, or longer than, SAAMI's specified maximum chamber size. If chamber headspace is excessive, the gun should be taken out of service until it has been inspected and repaired by a competent gunsmith. FIELD gages are slightly shorter than the SAAMI maximum in order to give a small safety margin. </div></div>


I am accustomed to using military headspace gauges and on The M16 family or rifles and there is only a nogo gauge and the bolt should not close at all on these gauges when we check. However using commercial gauges could very well be completely different from what I am accustomed to. This is why I went to the conclusion that I did.
I have to admit that the only commercial gauges I have used for head space have been a go gauge when doing barrels for my bolt guns and a pull through reamer for my M1A barrel I did recently. All were go gauges that the bolt had to shut on plus my one and half thousands shim I use. I don't like my headspace to be too tight as I shoot suppressed and have had issues with tight headspace and factory ammo after the barrel was good and dirty.