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Old December 16, 2019, 12:10 PM   #1
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Bolt group vs headspace

I hope that i am overthinking things but i have read that an ar upper and the bolt carrier group have to be “paired” to get the right head spacing. If this is true how can you buy a barrel one place and a “special” bcg another? I am in the process of building my first Ar so if someone could enlighten me I would like it.
Thanks for the replies and knowledge!!
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Old December 16, 2019, 01:01 PM   #2
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bolt carrier group have to be “paired” to get the right head spacing
I suppose that is technically true and you should check headspace before you shoot it. That's probably the most correct answer.

I've never done it.

If the bolt and barrel/extension are "in-spec", the headspace should be, also. Typically, I'll check the first several spent shell casing for signs of over pressure or other problems.
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Old December 16, 2019, 03:51 PM   #3
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Its not about the whole bolt carrier group. Of the components of a BCG,only the bolt matters to headspace.

If you bought a bolt/barrel matched to each other,it does not matter what bolt carrier you assemble it to.

The barrel extension is installed to the barrel. The barrel extension has the locking lug surfaces the bolt engages in battery. The bolt lugs resting on the bolt lug surfaces determine where the bolt breech face will be.

Headspace will be the distance from the breech face to the shoulder datum reference in the chamber. With an AR,the headspace lives in the assembly of three parts. The barrel,the barrel extension,and the bolt.

I had an experience with a barrel maker who had a little production glitch. I was lucky enough to be on the receiving end of it. Customer service was great,I was just told "Send it back to me" Another barrel was on the way.

I bought my barrel matched to a bolt.I returned the bolt,too. The replacement barrel came without a bolt.

OK. I had 3 AR bolts in my toolbox. My local Gunsmith's "No Go" headspace gauge was a "Go" with all three bolts. That barrel was sent back,too.

In the grand scheme of things,the design tolerances of nearly all US military small arms and most commercial guns call for the parts to assemble to be "in spec" . That does not mean we abandon headspace gauges and checking headspace. Yes,even on ARs

I don't care how many you assembled in your garage that "didn't have problems"

If you are going to build rifles,either buy headspace gauges and use them,or have a friendly local gunsmith check it for you.

That's one of the responsibilities of building a rifle.
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Old December 16, 2019, 05:03 PM   #4
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I've put together 2 uppers made up of barrels and bolts from different manufacturers. Checked both assemblies using a "no-go" gauge. Both checked out fine for function and accuracy. Bolts weren't "fitted/matched" to the barrel. I believe a matched bolt is a feature that costs more and is intended to make for a more accurate firearm.
I do know it adds to the cost but have no experience concerning the "accuracy" claims.
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Old December 16, 2019, 05:28 PM   #5
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In theory,you SHOULD be able to build about any relatively modern military or commercial rifle with the barrel,bolt,etc that come from the stockroom parts bins that are made to the tolerances on the blueprint.

Including a 1903 Springfield

But a riflemaker who has respect for his/her craft will check headspace on every firearm they build.

The AR mechanic will spend money on special vise jaws and barrel nut wrenches,etc,but What? $ 40 for a set of headspace gauges is too much?

Or just take your barrel assy and stripped bolt and go see your local gunsmith.

Buy some powder,brass,and bullets and ask about a quick headspace check.
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Old December 17, 2019, 02:28 PM   #6
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"..."in-spec"..." Who's 'spec'? Headspace is checked using proper headspace gauges, not switching 'uppers' until it's right.
Checking with just a No-Go does nothing.
$40 buys one of the gauges in a set. $87 buys a set from PT&G. Brownell's wants $66 for a 'Go' and 'No-Go'. The 'Field' gauge is not 100% necessary. Wasn't used by the CF at all.
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Old December 18, 2019, 06:27 PM   #7
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I've built 20+ ARs and have never checked headspace on any of them. And I've never had a problem. I also have wildcats. No headspace gauges available those calibers. I reload and always check the case from a new build. But I don't use an old/worn bolt with a different barrel.

If concerned about headspace, then order the AR barrel and bolt from the same vendor and make sure they'll check the headspace for you.

Last edited by ed308; December 18, 2019 at 08:52 PM.
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Old December 18, 2019, 08:10 PM   #8
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Its not a big trick to be enough of a mechanic to assemble an AR from parts.

Where the mechanic skills start to evolve into gunsmith skills includes standards,responsibilities,and a sense of doing things right.

I bought and returned an excessive headspace cut rifled barrel from an outfit that was known to have their uppers and rifles in the winners circle on a national level. They made things right without question,and I don't want to trash their reputation.I'd buy from them again.

But still,I KNOW I can spend near $400 for a top grade barrel and get one with excessive headspace because I have done it,and the reason I know is BECAUSE I CHECKED IT.

If you don't check them,you have no clue what friends and family are putting their face behind.

If a guy builds one AR without gauging it,well,maybe he isn't going to build more rifles. Maybe he doesn't need to buy gauges.
But at some point,there will be a local gunsmith or friend with gauges,and it can get checked.

When someone tells me "I built a hundred AR's and never checked headspace,what impresses me is that a set of headspace gauges would add about 70 cents to the cost of each rifle.

Like,the more you build,the less sense it makes to not have gauges.

If I can come up with a reamer for a wildcat,I can come up with a gauge.

I figure my rifles will be shooting after I'm dead,but the quality of my work will still be there.

Every rifle I build gets headspace checked.Nothing else will do.
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Old December 18, 2019, 08:18 PM   #9
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There is really no replacement for headspace gauges. I built an AK from a "kit" I ordered gauges for that one build.
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Old December 18, 2019, 10:04 PM   #10
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If you check with the proper gauge, you know. IF you don't you're guessing. Maybe its ok, maybe not.

I installed quite a few barrels in M16A1s in the 70s. The process is straightforward, you install the upper on the barrel, insert the proper gauge in the chamber and install the bolt carrier group. if the end of the bolt carrier protrudes beyond the end of the upper, its a fail. If its doesn't it goes out the shop door.

If it does fail, you swap the bolt (only) with one from the parts bin, and test again. You repeat this until it passes. If it fails with all the bolts in the parts bin, you replace the barrel and try the entire process again, after DX-ing the bad barrel.

That's what the Army did in the 70s. What you friendly neighborhood ASSEMBLER (not builder) does, I have no clue.
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Old January 14, 2020, 05:51 PM   #11
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Buy a barrel from a reputable manufacturer. One of the big boys, that have made barrels for decades, and can afford to maintain modern plant. Being "in-spec" actually allows for a certain amount of tolerance stacking. Smaller firms, new to the game are running lower volumes and most haven't "settled down" (thought they might have a great logo and web-site that would have you think otherwise). Don't shop for the cheapest decent barrel you can find either, it's the most important part of the build—bar none. Buy once, cry once—something that is CHF CL HPT MPI and never look back.
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Old January 15, 2020, 02:42 PM   #12
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I have headspace gauges for anything I buy a bcg for, go/no/field. I pull the ejector and check every time, no question there..

But the carrier aligment to the end of the upper is a perplexing detail for me. Very minor, but all a little different - some a hair out or inside. But then the upper may have a hair difference where it meets the lower even. Rifles all run great.

I've never seen a spec on this just a general statement it should line up. My feeling is if it was that important there would be a gauge for it. Would be very interested in more detail on this topic and what problems result.
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Old January 15, 2020, 03:15 PM   #13
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Maybe I do it wrong,but I check with a strpped bolt. No bolt carrier parts. Just the bolt,the gauge,and the barrel/extension assy.

I put the gauge in the chamber,an see if my thumb and forefinger will spin the bolt through the locking lugs.I do it before I assemble the barrel to the upper.
Of course,you can check it again later.

That's pass/fail for headspace. How the rest of the tolerances stack up may or may not be an issue,but its not headspace.

Last edited by HiBC; January 15, 2020 at 05:31 PM.
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Old January 15, 2020, 05:15 PM   #14
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I've always purchased upper sets, and have keep the barrel and bolt itself together. Otherwise headspace verification is needed.

Correct headspace of a bolt and barrel purchased separately cannot be assumed and must be verified to ensure safety.

Last edited by BBarn; January 15, 2020 at 05:28 PM.
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Old January 15, 2020, 05:37 PM   #15
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HiBC is doing it right. Getting everything else out of the way is the only thing that allows a light enough touch to close the bolt that it will still let you feel if there is any interference with the GO gauge. If any amount of spring pressure or other force is involved in the bolt closing, it can easily stretch the chamber slightly so you don't know it is actually a little tight until you get a feed failure. The only situation in which you don't need to check headspace is when using an accurate headspacing pull-through reamer for final chamber size, but you still want to check its work the first time you use it to be sure the bushing thickness is correct. PTG makes these reamers in .223 Rem.
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Old January 16, 2020, 09:02 AM   #16
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Because the bolt rotates within the carrier which is timed to the gas port--I still do headspace checks with the bolt in the carrier in the receiver in addition to the hand check before assembly. I've had two barrels over the years with faulty extension installs.
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