AR-15 5.56 Headspace gauging

[Zartan]

So I'm going to be assembling my upper soon. When gauging, do I need the go/no-go or just the go? Also, the barrel is chambered 5.56 which means I should buy the 5.56 gauge and not the .223. Correct?

TIA

[a308garand]

Go gauge = Bolt should close on action. Ensures the headspace is not too short. Things are looking good for the bolt & barrel not being worn out.

No Go gauge = Bolt should not close on action. Ensures the headspace is not too long. The closer the gauge gets to allowing the bolt to close in the action, the more wear is present on the bolt & barrel combination. If the bolt closes with some resistance but does not close on the Field gauge, the rifle is still safe to shoot...just needs parts replacement sooner.

Field gauge = Bolt should not close on action. This is the last gauge to ensure safety, if your action closes on the No Go, it must not close on the Field gauge. When the bolt closes on this combination, its time to replace parts.

Save on shipping and just buy all three gauges at once. If they do not have a cut-out for the ejector, you will need to competely strip the bolt of the extractor and ejector to get an accurate measurement.

[FeuerFrei]

AR barrels are head spaced by manufacturer when barrel is fitted to the extension. No need to head space a new barrel/bolt assembly.
Do buy the gauges if you want. Not really necessary.

[ar15barrels]

AR barrels are head spaced by manufacturer when barrel is fitted to the extension. No need to head space a new barrel/bolt assembly.
Do buy the gauges if you want. Not really necessary.

Gauging the specific barrel and bolt yourself is the only way to ensure that the parts actually ARE in spec.
Many people feel more comfortable knowing a fact rather than assuming someone else did their job correctly...

[Spyguy]

Three questions:

1. What is the purpose of the 5.56 No-Go gauge (for AR uppers)? It seems you would only need the Go gauge and Field gauge; the No-Go gauge seems superfluous.

2. I assume that the Mil-Spec for barrel extensions and bolts have enough design tolerance to allow stacking errors: i.e., if one manufacturer is assembling the barrel extension at one extreme of the spec, and another manufacturer is machining the bolt at the other extreme of the spec, that together they will still be within the allowable headspace dimensions. Is this correct? Still, I would especially want to headspace a chamber when using parts from different manufacturers for an AR build.

3. What brand or set of gauges are recommended? I searched Brownells, and the only 5.56 Go/No-Go gauges they offer are by Clymer. But the only 5.56 Field gauge they carry is Brownell's own brand (and is sized at 1.4736, which is the maximum). I'd prefer a set that is made by the same manufacturer and with a Field gauge size of no more than 1.4730".

[kcstott]

Have a field gage is nice but not necessary for a user builder. A professional on th either hand should have them all no question about it.
Personally I believe you should have a go, no-go set and leave the field gage to the armors in service. You need to understand that just because the the action will not close on a field it tells you nothing about the condition of the headspace. it only tells you if your weapon is still safe to shoot. but how close is it?? Is it still tight?? or will it close on a no-go?? you will never know without the gages.

[Spyguy]

Have a field gage is nice but not necessary for a user builder. A professional on th either hand should have them all no question about it.
Personally I believe you should have a go, no-go set and leave the field gage to the armors in service. You need to understand that just because the the action will not close on a field it tells you nothing about the condition of the headspace. it only tells you if your weapon is still safe to shoot. but how close is it?? Is it still tight?? or will it close on a no-go?? you will never know without the gages.

But what if it closes on the no-go? From your reply, I'm guessing that should never occur for a new assembly. But what about a rifle with several thousand rounds through the barrel and then the bolt breaks? Is it possible that the replacement bolt will close on the no-go gauge, but still be safe (less than field gauge dimensions)?

[ar15barrels]

AR-15 5.56 Headspace gauging

But what if it closes on the no-go? From your reply, I'm guessing that should never occur for a new assembly. But what about a rifle with several thousand rounds through the barrel and then the bolt breaks? Is it possible that the replacement bolt will close on the no-go gauge, but still be safe (less than field gauge dimensions)?

It depends on the gauge dimensions.
It's perfectly acceptable for a 5.56 spec barrel to close on a 223 no-go gauge.

Have you seen my collection of headspace gauge dimensions?

https://www.ar15barrels.com/data/headspace.pdf

[kcstott]

But what if it closes on the no-go? From your reply, I'm guessing that should never occur for a new assembly. But what about a rifle with several thousand rounds through the barrel and then the bolt breaks? Is it possible that the replacement bolt will close on the no-go gauge, but still be safe (less than field gauge dimensions)?

Here's the deal you are thinking about this way to much. Unless you are using your rifle for actual combat use a field gage is going to be of no us. If my rifle closes on a 5.56 no go I'm changing the barrel. Headspace is the first thing to effect accuracy in internal ballistics

It depends on the gauge dimensions.
It's perfectly acceptable for a 5.56 spec barrel to close on a 223 no-go gauge.

Have you seen my collection of headspace gauge dimensions?
www.ar15barrels.com/data/headspace.pdf

Exactly.

[Spyguy]

It depends on the gauge dimensions.
It's perfectly acceptable for a 5.56 spec barrel to close on a 223 no-go gauge.

Have you seen my collection of headspace gauge dimensions?
www.ar15barrels.com/data/headspace.pdf

Yes Randall, I've saved your headspace page and studied it. And your response just highlighted my concerns: if I have only a Go and No-Go gauge, and the bolt closes on both, then what have I learned? All I know is that the chamber is not too short, but that it may be too long... or maybe not. Which means I won't know if it's actually safe or not.

[Spyguy]

Here's the deal you are thinking about this way to much. Unless you are using your rifle for actual combat use a field gage is going to be of no us. If my rifle closes on a 5.56 no go I'm changing the barrel. Headspace is the first thing to effect accuracy in internal ballistics

My military service days are well over. So hopefully, no combat use. But if I purchase a new 5.56 rifle from a company that builds to Mil-Spec under government contract, is it possible that the rifle could close on the No-Go gauge?

What if I assemble a new barrel from one manufacturer and a bolt from another? Is it possible that, even if new, that the combination could close on a No-Go?

In either event, it appears your advice would be to have the headspace corrected, even if the parts are new and still safe per a field gauge. Is that the correct answer?

[a308garand]

My military service days are well over. So hopefully, no combat use. But if I purchase a new 5.56 rifle from a company that builds to Mil-Spec under government contract, is it possible that the rifle could close on the No-Go gauge?

What if I assemble a new barrel from one manufacturer and a bolt from another? Is it possible that, even if new, that the combination could close on a No-Go?

Anything is remotely possible, but in real world terms, brand new parts made by reputable makers will pass inspection for assembly. That means they will check ok using both "Go"&"No Go" for the initial headspace measurements.
On the off chance that something was not built properly and it slipped past quality control, use the company's return warranty service to get the correct parts. Everything from an over reamed chamber or out of spec chamber extension could cause a good bolt to fail inspection. A bolt with short or long lugs could also cause a good barrel to fail inspection. Return the bad parts for replacement.

In either event, it appears your advice would be to have the headspace corrected, even if the parts are new and still safe per a field gauge. Is that the correct answer?

If the headspace is good, there is no correction needed. Barrels are more expensive than bolts, so as long as the barrel is in good shape, you swap bolts to correct a condition. Helps a ton to have a bucket of spare bolts, not something the average guy has in their garage. If the barrel is worn down, its cost effective to just replace both bolt & barrel with new parts...may even involve the upper receiver as well.
When you build your own rifle, you take on all the responsibilities of inspection and fitting that the factory normally would do. Starting with factory new parts from reputable makers, who warranty their product, is a big first step towards having little to no issues. Buying unknown quality stuff to save a few bucks can bite you in the long run.

This topic sure came back from the wayback machine

[PolishMike]

I use Ptg gauges. They make top notch stuff

[kcstott]

My military service days are well over. So hopefully, no combat use. But if I purchase a new 5.56 rifle from a company that builds to Mil-Spec under government contract, is it possible that the rifle could close on the No-Go gauge?

What if I assemble a new barrel from one manufacturer and a bolt from another? Is it possible that, even if new, that the combination could close on a No-Go?

In either event, it appears your advice would be to have the headspace corrected, even if the parts are new and still safe per a field gauge. Is that the correct answer?

What a308garand said and again you are way over thinking this.

If it's a new assembly and it's out of spec the barrel is returned to the maker for a replacement. There is nothing you can do to correct a long head place on an AR at least nothing cost effective. Short chamber is different you can use a pull through reamer. But again I'm sending stuff back

As far as tolerance staking goes NO there is not enough tolerance on both parts to stack and cause a problem if both part are within spec.

Reading the Ordinance print from 1970 th barrel extension locking lugs face is listed as a ground finish after heat treat with the dimension being +/-.001
I don't have a ordinance print of the bolt but I would assume the tolerance from bolt face to lug surface is also =/-.001" so in a worse case scenario you'd be .002" and that is only one half to one third the total from go to no go dimension

Again using mil spec parts you are good to go. If you want a field gage get it. all gages call out a spec of either within .0002" or +/-.0002" in any event it's nothing to worry about getting gages from different manufactures as long as you know the gaging distance called out on the gage.

[Spyguy]

Thanks to all for the thoughtful and informative replies. Everything is more clear now.

This topic sure came back from the wayback machine

There will always be new people learning new things. I searched for posts on headspacing and this was one of the more informative threads. Instead of starting a new thread, I think it's better to add to an existing good one for the benefit of others who may have the same questions as I.