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Why do I need headspace gauges for an AR build? Login/Join 
Freethinker
Picture of sigfreund
posted
First, let me say that I have headspace gauges and so whether I should buy them isn’t the question.

When I replaced an AR-15 barrel for the first time the one I ordered came with the barrel extension already attached. As I was pondering its installation it also dawned on me that AR barrels must be indexed to align with the gas block and of course the fit between the barrel extension and bolt is fixed by the dimensions of those two parts.

My question therefore is this: What would one do if the headspace gauges indicated there was a problem? With the AR barrel and bolt design, what can be adjusted if the headspace is too great or too short? Are the gauges just to confirm it’s not necessary to send the new barrel and extension back for a refund?

I’m sure that this is one more question that will have people scratching their heads and wondering, “Really? He doesn’t know that?” but it’s something I’ve never seen discussed and if improper headspace with AR barrels were a common problem, I would expect to have seen complaints about it.

(I did find one Internet article that claims bad headspace is extremely common in AR barrels, perhaps as many as 135% of them—well, that’s an exaggeration of course, but now I’m curious how many people actually experience problems or find that their new barrels are bad, and then what they do to correct the problem.)

Thanks for all informative replies.




“A man hears what he wants to hear and disregards the rest.”
— Simon & Garfunkel, The Boxer, 1970
 
Posts: 41488 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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They're used as a confirmation of spec. If out of spec there's nothing to adjust; try a new bolt. I've not come across any ARs that had excessive headspace, but I've heard of them Big Grin


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Now an FFL licensee, working on SIGs and other assorted firearms. My email is in my profile.
 
Posts: 8458 | Location: UT | Registered: December 05, 1999Reply With QuoteReport This Post
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Stacked tolerances would be my answer. I've had a bolt that wouldnt fit a barrel/extension though they were both in "spec". Especially with the variety of makers of all AR parts and the lack of QC by some companies its not a terrible idea.
 
Posts: 2180 | Location: Pnw | Registered: March 21, 2009Reply With QuoteReport This Post
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Picture of Rustpot
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This is one of the reasons I'm becoming more and more of an advocate of buying a rifle vs. building for a beginner. And this is coming from someone who's been "building" ARs for the better part of a decade, the first a PSA with a claw hammer, a c-clamp, and a $15 armorer wrench.

What would one do if the headspace gauges indicated there was a problem?

You have a few options.

The easiest for most people would be to return the components. Ideally they're from the same company, often not when dealing with builds. Someone just started offering a barrel+bolt package (not BCG, just the bolt) to avoid this very issue. You could have someone check them out, but there's cost, time to find a competent smith/armorer, figure out which is bad, THEN ultimately return it to the manufacturer.

For the more adept with many guns you can go through a process of elimination and start swapping bolts and barrels and gauging them until you figure out what part you have is bad. Could be both, yielding false positives, could be indeterminable with this method, depending on the exact issue.

For the most advanced they'd have the tools to further diagnose. Confirming bolt measurement vs. TDP, using more barrel gauges and examination methods to check for issues that can be impacting headspace.

Keep in mind reading headspace with normal gauges in the AR requires disassembling the bolt. Extractor comes off easy enough. The ejector can be a pain. Especially if you don't have spare roll pins and ejectors to replace those you've broken or launched into orbit.

Since I got my headspace gauges I've checked close to a dozen guns. I've had one instance of short headspace, using a toolcraft/PSA BCG and a BA barrel. The bolt would baaaaarely pass .223 GO. That barrel passed headspace with 5 other BCGs from SOLGW, Colt, RRA, and Sionics, and the unmarked one that was in it. I concluded it was the toolcraft/PSA bolt. It had other issues, besides, and needed to be returned.



So why care? The only way the gun really becomes dangerous to the operator is with bad headspace.

Non-catastrophic large/long headspace will typically damage the brass from stretching, tearing, or otherwise deforming it from giving it too much room. It can cause cycling issues with extraction, and pop primers if the brass isn't held against the bolt face.

Small/short headspace that still allows the bolt to lock closed results in function issues, overpressure in the chamber that flattens primers and etches the bolt face after it's blowing by that primer.

Too short and combine it with a long firing pin, or short bolt tail (which are problems unto themselves), and you can have an out of battery detonation.

Either means you pop a lot of primers into your action at best, and those buggers are one good way to lock up a rifle in a hurry.
 
Posts: 5822 | Location: Romeo, MI | Registered: January 03, 2009Reply With QuoteReport This Post
Freethinker
Picture of sigfreund
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Thanks, Rustpot, for all that, and to the others who basically confirmed my own analysis of the question I had.




“A man hears what he wants to hear and disregards the rest.”
— Simon & Garfunkel, The Boxer, 1970
 
Posts: 41488 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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I would never use a new barrel or bolt without checking it. I have had quite a few that were not good, at least in combination with the other part. What you do is swap parts till its ok or send the offending part back. That at least is the TM view of the world and what I do. There really isn't anything dangerous about shooting an AR except this, I treat it very seriously.


“So in war, the way is to avoid what is strong, and strike at what is weak.”
 
Posts: 8450 | Registered: October 14, 2004Reply With QuoteReport This Post
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Picture of Expert308
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If the headspace is short then I supposed you could take the barrel to a smith and have them ream the chamber a little deeper. Other than that, swapping bolts or returned the parts is about all you can do.

Another approach is to buy your barrel un-chambered and then supply a bolt to the smith you're going to have chamber it for you. They can ream the chamber to be a precise match to your bolt.
 
Posts: 5888 | Location: Portland, OR | Registered: February 12, 2007Reply With QuoteReport This Post
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Picture of kimberkid
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I bought a headspace gauge when I put my first AR together ... That was the only time I ever used it and I have no idea where it even is any more. I've moved 4 times in the last 40 years but I can account for every other tool and reloading part, but that headspace guage, I have no idea.

The only bolt that wouldn't fit in another AR is a KAC SR-15. I'd read that they are proprietary ... and they are.


If you really want something you'll find a way ...
... if you don't you'll find an excuse.

I'm really not a "kid" anymore ... but I haven't grown up yet either Wink
 
Posts: 5199 | Registered: January 11, 2007Reply With QuoteReport This Post
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We are not talking about does the bolt 'fit' as in install, I've actually never heard of an AR bolt that didn't fit (I suppose when you build millions of things that a small percentage of them are just wrong, so nobody needs to tell me you got one from PSA that the lugs were wrong). The issue is it matched properly to the barrel you intend to use it in. And to know that you need a headspace guage. Why anyone would avoid this simple and needed step for a gun they built eludes me. Sure the vast majority of them are fine, but I don't want the one that isn't to be mine, all to save forty bucks.


“So in war, the way is to avoid what is strong, and strike at what is weak.”
 
Posts: 8450 | Registered: October 14, 2004Reply With QuoteReport This Post
semi-reformed sailor
Picture of MikeinNC
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Just borrow a set from someone nearby.

No need to buy one.



"Violence, naked force, has settled more issues in history than has any other factor.”
― Robert A. Heinlein, Starship Troopers

 
Posts: 6896 | Location: Texas! | Registered: October 07, 2006Reply With QuoteReport This Post
Freethinker
Picture of sigfreund
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quote:
Originally posted by MikeinNC:
Just borrow a set from someone nearby.

No need to buy one.


Thank you.

quote:
Originally posted by sigfreund:
First, let me say that I have headspace gauges and so whether I should buy them isn’t the question.


Smile




“A man hears what he wants to hear and disregards the rest.”
— Simon & Garfunkel, The Boxer, 1970
 
Posts: 41488 | Location: 10,150 Feet Above Sea Level in Colorado | Registered: April 04, 2002Reply With QuoteReport This Post
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When checking headspace are you stripping your bolt first?
 
Posts: 1494 | Location: Spokane, WA | Registered: June 23, 2008Reply With QuoteReport This Post
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I'm quite sure this isn't the best advice , but it has been what I do.

After I have the gun assembled, I take it to the range, tie it to an old tire , attach a 10 foot string to the trigger and fire a few of rounds.
I then examine the spent brass for signs of excessive headspace or pressures.

Take this fwiw...
 
Posts: 915 | Location: Idaho | Registered: October 21, 2007Reply With QuoteReport This Post
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What do we consider an AR build?

I bought a BCM complete lower, Daniel Defense upper and a DD bolt. Do I need to check the headspace? I put it all together and it goes bang everytime.


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Posts: 4327 | Location: Pittsburgh, PA, USA | Registered: February 27, 2001Reply With QuoteReport This Post
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Picture of kimberkid
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quote:
Originally posted by mike28w:
I'm quite sure this isn't the best advice , but it has been what I do.

After I have the gun assembled, I take it to the range, tie it to an old tire , attach a 10 foot string to the trigger and fire a few of rounds.
I then examine the spent brass for signs of excessive headspace or pressures.

Take this fwiw...

Being a reloader I pay close attention to my brass too


If you really want something you'll find a way ...
... if you don't you'll find an excuse.

I'm really not a "kid" anymore ... but I haven't grown up yet either Wink
 
Posts: 5199 | Registered: January 11, 2007Reply With QuoteReport This Post
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quote:
After I have the gun assembled, I take it to the range, tie it to an old tire , attach a 10 foot string to the trigger and fire a few of rounds.
I then examine the spent brass for signs of excessive headspace or pressures.

Are you nuts? All to avoid buying a simple low cost gauge and using it? really? And exactly what does that tell you? Its not often I read stuff that makes me shake my head on sigforum but this does. Guess what you've been here since 2007. Email me an address and I will send you a headspace gauge. free.


“So in war, the way is to avoid what is strong, and strike at what is weak.”
 
Posts: 8450 | Registered: October 14, 2004Reply With QuoteReport This Post
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quote:
What do we consider an AR build?

The lower matters not. If you bought a complete upper from a top tier mfg. like DD(complete meaning barrel, upper, bolt carrier and bolt) they will have checked the headspace.


“So in war, the way is to avoid what is strong, and strike at what is weak.”
 
Posts: 8450 | Registered: October 14, 2004Reply With QuoteReport This Post
Master of one hand
pistol shooting
Picture of Hamden106
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So swapping barrel and bolt from an A2 NM upper to an A4 upper (all RRA) should be ok. But I should check HS anyway.



SIGnature
NRA Benefactor CMP Pistol Distinguished
 
Posts: 5248 | Location: Duckburg, OR | Registered: September 01, 2001Reply With QuoteReport This Post
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Not sure I follow exactly the question, but if you have a barrel and bolt that were headspaced correctly and are in spec, moving both of them to another upper is not an issue assuming correct assembly. the AR bolt headspaces in the barrel extension and the upper is just along for the ride.


“So in war, the way is to avoid what is strong, and strike at what is weak.”
 
Posts: 8450 | Registered: October 14, 2004Reply With QuoteReport This Post
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quote:
If you bought a complete upper from a top tier mfg. like DD(complete meaning barrel, upper, bolt carrier and bolt) they SHOULD have checked the headspace.


More the case.
Anecdote alert:
A guy here bought a complete AR rifle. It was reliable and extremely accurate for the type. But he dropped a fired case into one of those RCBS gauges and was horrified to see "excess headspace." He demanded and got a replacement barrel. The joke was on him, it was not nearly as accurate as the sloppy original.
 
Posts: 2843 | Location: Florence, Alabama, USA | Registered: July 05, 2001Reply With QuoteReport This Post
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