AR headspacing question

[BigK]

Is it a big deal or not?

 

A co-worker bought a go gauge (Forstner 5.56 NATO Headspace Gage - 1.4636" plus .0003" minus zero - minimum gauge), just to be on the safe side after having some other trouble with his build. His bolt won't close and lock on the go gauge, so he's concerned to shoot it. Since he and I bought the same barrel (16" Radical Firearms 1:9, 5.56, M4 carbine-length) on sale at Primary Arms, he asked me to check my headspace. Turns out mine won't close on the gauge either.

 

Unlike him, I had already taken mine out and shot it first chance I got after finishing the build. It chambered and fired fine and now that I got the gas block adjusted like I want it, it also cycles perfectly. He's till reluctant to shoot his, even though mine shoots fine and won't close on the gauge either. Both of ours lack about 3/16" to 1/4" closing on the gauge (I'll post pics soon).

 

I'm curious as hell what to make of this go gauge issue. Do you think his is safe to shoot? Should I stop shooting mine? Think the gauge is counterfeit and/or wrong?

[mikegideon]

If it chambers factory ammo, and there's no evidence of excessive pressure on the fired brass, you are fine. 3/16" is WAY out of spec, and indicates that the gauge is faulty, or not dropping into the bolt correctly. Try pushing the bolt closed with the forward assist. I wouldn't bang on it. Just give it a thumb push and see if it snaps into the bolt.

 

The big danger with headspace is when the no go drops in easily. That's the recipe for a kaboom. no matter what, the fired brass will tell the whole story. I have one of these...

 

http://www.midwayusa.com/product/477756/rcbs-precision-mic-223-remington

Edited October 27, 2014 by mikegideon

[Clod Stomper]

If that gauge is correct, there is no way your rifle would chamber a round if it lacked 3/16".

 

Did you take the extractor and ejector out of the bolt to check the headspace?

[gregintenn]

Is the rim of the gauge getting behind the extractor?

 

I think one is supposed to remove the ejector and extractor in order to properly use a headspace gauge; at least that is the procedure with a bolt rifle.

Edited October 27, 2014 by gregintenn

[mikegideon]

Extractor is easy. The ejector isn't. I have a specialized tool for that. you can find your true headspacing by measuring the shoulder bump on fired brass with the RCBS micrometer.

[mikegideon]

Is the rim of the gauge getting behind the extractor?

 

I think one is supposed to remove the ejector and extractor in order to properly use a headspace gauge; at least that is the procedure with a bolt rifle.

 

It can't be with the 3/16" error

[gregintenn]

THe way an ar works, it seems to me like one should be properly headspaced as long as the barrel and bolt are built to spec. I've never checked one with gauges.

Edited October 27, 2014 by gregintenn

[mikegideon]

THe way an ar works, it seems to me like one should be properly headspaced as long as the barrel and bolt are built to spec. I've never checked one with gauges.

 

Bingo! I have lots of AR and 223 ammo tools. Headspace guages... nah

Edited October 27, 2014 by mikegideon

[BigK]

Is the rim of the gauge getting behind the extractor?

 

I think one is supposed to remove the ejector and extractor in order to properly use a headspace gauge; at least that is the procedure with a bolt rifle.

 

I tried it with the extractor in place and without. With the extractor in place it pulled the headspace gauge out of the chamber when I removed the BCG. So, I'd say it's getting behind the extractor. Here's a couple of pics to show just how far out of battery it is with the bolt as far forward as it'll go. EDIT: looking at it from underneath, the bolt lacks a mm or less from being able to rotate under the lugs, which would allow the carrier to slide forward when it rotates.

 

 

Edited October 27, 2014 by BigK

[peejman]

I tried it with the extractor in place and without. With the extractor in place it pulled the headspace gauge out of the chamber when I removed the BCG. So, I'd say it's getting behind the extractor. Here's a couple of pics to show just how far out of battery it is with the bolt as far forward as it'll go. EDIT: looking at it from underneath, the bolt lacks a mm or less from being able to rotate under the lugs, which would allow the carrier to slide forward when it rotates.

 

 

Yes, the extractor should be removed when checking headspace.  1mm = 0.039" which may as well be a mile as far as headspace is concerned.  I've never headspaced an AR, but I have to think you have the wrong gauge.  Does the gauge fit in the chamber? 

[BigK]

I feel like I did a poor job of explaining that, sorry. Let me try again:

 

With the bolt all the way forward in the carrier (cam pin completely forward), the bolt face engages the gauge. Because the lugs on the bolt can't go behind the lugs in the barrel, the bolt cannot rotate into position. That explains how far back the bolt carrier is. So when I say this looks like 3/16" or maybe even 1/4", as you can see in the pics, this distance is the entire distance the carrier moves as the bolt closes.

 

This is NOT however a fair estimate of how far out of spec it is. I don't have a pic from inside the receiver, but if I did, you could see how close it is to fitting. The bolt lugs need less than a mm to be able to rotate behind the barrel lugs. Once behind the barrel lugs, the bolt could rotate and the carrier would come forward and it wouldn't look like it's so far out of spec.

 

That make sense? Am I right about the distance needed for the lugs to lock being the real thing to measure here?

[mikegideon]

 

 

That make sense? Am I right about the distance needed for the lugs to lock being the real thing to measure here?

 

Yes, and you're right. The last bit of carrier travel is to rotate the bolt.

[Oh Shoot]

Is it a big deal or not?

 

A co-worker bought a go gauge (Forstner 5.56 NATO Headspace Gage - 1.4636" plus .0003" minus zero - minimum gauge), just to be on the safe side after having some other trouble with his build. His bolt won't close and lock on the go gauge, so he's concerned to shoot it. Since he and I bought the same barrel (16" Radical Firearms 1:9, 5.56, M4 carbine-length) on sale at Primary Arms, he asked me to check my headspace. Turns out mine won't close on the gauge either....

 

Perhaps it's actually a no-go gauge?

 

- OS

[BigK]

Perhaps it's actually a no-go gauge?

 

- OS

No kidding.

 

That's exactly what I thought, but it says MINIMUM right on the side of the gauge plain as day. Doesn't mean it's not mislabeled, but isn't Forster a reputable manufacturer?

[Oh Shoot]

No kidding.

 

That's exactly what I thought, but it says MINIMUM right on the side of the gauge plain as day. Doesn't mean it's not mislabeled, but isn't Forster a reputable manufacturer?

 

And both extractor and ejector are removed?  I've never used one, but every guide I've read says neither should be present.

 

- OS

[mikegideon]

And both extractor and ejector are removed?  I've never used one, but every guide I've read says neither should be present.

 

- OS

 

Yep BigK. Remove the ejector, and we'll be meeting at the Waffle House for sure. You will need to borrow my "puttin' the ejector back in" tool :)

 

http://www.midwayusa.com/product/231625/schuster-ar-15-bolt-extractor-and-ejector-disassembly-tool

Edited October 27, 2014 by mikegideon

[Oh Shoot]

Yep BigK. Remove the ejector, and we'll be meeting at the Waffle House for sure. You will need to borrow my "puttin' the ejector back in" tool :)

 

http://www.midwayusa.com/product/231625/schuster-ar-15-bolt-extractor-and-ejector-disassembly-tool

 

Again, not needed to do one myself, but doesn't look bad at all, here removed,  and replaced with nothing more than using a cartridge as aid?:

 

https://www.youtube.com/watch?v=IWTpPMYW-CA

 

- OS

[mikegideon]

That's a roll pin for the ejector. But, everybody needs to try it once without the tool :). It can be done. I've done the cartridge thing before.

[BigK]

Hell no I didn't take the ejector and extractor off of my bolt, I'm WAY too lazy for that crap. LOL Plus I was already happy that mine shot. We used his fully disassembled DPMS bolt and carrier when we measured the headspace. I tried mine too for grins, though, and got the same results.

I still don't see why anyone would make a gauge that can't accurately measure headspace without disassembling the bolt. You don't shoot without an extractor and ejector, so why should they make a gauge that requires them to be removed? Furthermore, if the fully assembled BCG closes on 3 or 4 different brands of ammo, I don't see what the gauge shows you that the ammo doesn't show you. Edited October 27, 2014 by BigK

[mikegideon]

Again... if you don't have enough headspace, the bolt just won't close. Too much is the dangerous condition.

[peejman]

I feel like I did a poor job of explaining that, sorry. Let me try again:

 

With the bolt all the way forward in the carrier (cam pin completely forward), the bolt face engages the gauge. Because the lugs on the bolt can't go behind the lugs in the barrel, the bolt cannot rotate into position. That explains how far back the bolt carrier is. So when I say this looks like 3/16" or maybe even 1/4", as you can see in the pics, this distance is the entire distance the carrier moves as the bolt closes.

 

This is NOT however a fair estimate of how far out of spec it is. I don't have a pic from inside the receiver, but if I did, you could see how close it is to fitting. The bolt lugs need less than a mm to be able to rotate behind the barrel lugs. Once behind the barrel lugs, the bolt could rotate and the carrier would come forward and it wouldn't look like it's so far out of spec.

 

That make sense? Am I right about the distance needed for the lugs to lock being the real thing to measure here?

 

 

The difference between a "go" and a "nogo" is only 0.010"... that's roughly 3 sheets of paper thick.  Further reading tells me that the only headspace gauge you can use without removing the extractor and ejector is the field gauge. 

 

 

On a related note.... the pic below is from Brownells....

 

 

 

 

You're advertizing rusty headspace gauges?  Really?  :squint:   Speaking of stupid marketing... :surrender:

 

http://www.brownells.com/gunsmith-tools-supplies/measuring-tools/headspace-gauges/no-go-5-56-nato-sku184000036-40876-51827.aspx