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"If you're going to be crazy, you have to get paid for it, or else you're going to be locked up." Dr. Hunter S. Thompson

Wednesday, March 3, 2021

Headspace

Greetings Me Droog N Droogettes!
OK per the fans out there: Headspacing an AR.
This's a bit deep so bear with me. I'll try not to fuck it up. Headspacing is defined as "headspace is the distance measured from a closed chamber's breech face to the chamber feature that limits the insertion depth of a cartridge placed in it."  Headspacing is critical in that if it ain't right, bad things can happen.

To check the headspace of a weapon, a set of precision gauges are used.  Now, these things aren't too expensive, and as such, the 'regular guy' can afford them, but mostly it's guys like me that keep them on hand.  

Now, First things first, the weapons have to be clean.  Like clean-clean.  No lube.  Lube can fuck up the results of a gauging. You need the bolt clean, as well as the chamber.  It was a stone bitch sometimes when we'd show up to do an Arms Room, and the weapons were dirty nasty.  Not too often, but when it did happen, we'd have the First Shirt call up a draft a bunch of doodz to clean 'em, lest we write 'em up for it.  

Now, as far as owning gauges.  It's a 50/50 situation with building ARs.  On the one hand, 99% of the rifle barrels out there are pre-set so to speak, meaning the barrel is gauged at the factory when the barrel is inserted and screwed down onto the feed ramp nut.  Headspacing is supposed to be checked before it leaves the factory.  This is done by inserting a "Go" gauge, which allows proper locking of the bolt to the chamber.  The "No-Go" gauge is used to check the bolt to insure that it does not properly lock.
Now, for the DotMil, we did it a bit different.  It still was functional, but we didn't have the time to tear the bolt head apart like a lot of the online guides say.  They show that the bolt is stripped down of the extractor, and extractor pin/spring.  When we were gauging an Arms Room, it was upward of 500 rifles usually, nevermind a few hunnerd SAWs, M240s and 50s, add on the Mk19s and we had to assembly-line the process pretty quick.

What we did was scissor the rifle open, remove the bolt and drop the "Go" gauge into the chamber.  We'd then re-insert the bolt til it was resting on the gauge.  Using the slightest pressure on the rear of the BCG, we'd push down on the BCG, getting the bolt to cycle into the chamber with the gauge.  It was a lot of 'feel' coming into play... after a while it was second nature to know a 'good' lock versus a 'bad' lock.  A bad lock as being defined as a type where as soon as the bolt rested on the gauge, it would lock in on it's own from the weight of the BCG... that sometimes indicated bad , missing or worn gas rings.  The rifles that failed the "Go" gauging were put aside for closer examination AFTER we got all the others knocked out.

Now, to the precision factor: The Proper “Go” Dimension for 5.56 is 1.4646 is actually the proper “go” dimension for a 5.56 chamber. That’s what Colt uses.  As well as my old DotMil Gauges.  Now another side note:  We had to have our gauges 'specc'd' at the DotMil lab in Maryland yearly.  They'd take quite a beating so to speak, and when you're measuring at the 4th decimal point for tolerances, you gotta keep them safe and not bang 'em around.  We had 3 sets.  One was ALWAYS out getting specc'd as it took forever as there's only ONE DotMil lab that can recert the gauges.  Two for use as there were two of us doing the job, and one on rotation at the lab.  When the set came back fromt he lab, we'd ship out the one that needed it next.

Now, commercial gauge(s)... problem is most companies, if you look at the gauge, it will say “5.56 NATO” on it and they are actually using the 1.4636 spec, which is closer to the SAAMI specification for .223.  Not -too big- a deal, BUT some folks get really really picky.

Me?  If'n it shewtz widdout blowing up?  I'm good.

Now, to review
Now, the field gauge?  That's used if the tolerance are slightly out-of-spec.  Meaning over time, the bolt and receiver will wear, the bolt and lugs compress, and the receiver may stretch, all causing the headspace to gradually increase from the factory specifications measured by the "go" and "no-go" gauges.

I personally don't use a field gauge.
The other gauges we used was the gas key insert, that measured the wear on the gas key, and the Straight Barrel Gauge, which was a highly specc'd cleaning rod for all intensive purposes.   It was to measure if there was a bend, even in the slightest in the barrel.  Surprisingly, there were quite a few that this gauge found... then again, not too surprising as the DotMil rifles have been abused like a rented mule in the past, as we all can well imagine.  Drop the rod down the barrel, and if it hung up -at all-, wash rinse repeat.  If it happened 3 times?  New barrel time.  To get the gauge out usually shaking the shit out of the upper is all it took, and making sure a nice big soft towel was on the bench to catch the gauge.

In fact, any gauging you do, don't forget your towel!
So, that's about it for now... I'm sort of a bit out of it today... GranBebe is over for the week.... apparently OtherGrans needed a break.  I can see why too... OMG she's nonstop.  So, More Later I Remain The Intrepid Reporter
Big Country





8 comments:

  1. As usual, you deliver up some good to know stuffs for those assembling a new one, or replacing older parts. Always appreciaate you sharing the skillz, BC !!

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  2. If you want to see someone doing these tests, check out Chad's School of the American Rifle YouTube channel.

    https://youtube.com/channel/UCaUOYvlDrZh5LWILrVDUM4w

    To skip the bolt disassembly, his headspace gauges have the "rim" removed.

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  3. "...a highly specc'd cleaning rod for all intensive purposes."
    How about non-intensive purposes? Lackadaisical purposes?
    --Tennessee Budd

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  4. good right up, gave me something to think about.... I'm seeing a 006 difference in brownells 5.56 go/nogo gauges 1.4636 and nogo 1.4696 is that right? 223 is closer tolerance, only 003 difference.. I use to be a machinist (not in the gun world) and 6ths is quite a bit.. should I buy the tighter tolerance g/ng gauges for my tool kit?

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  5. Really clean chamber? No problem. Chlorine-Free Brakleen gets it clean like nothing I've ever used. Wish I would have known about it when I was in.

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    1. I used carb cleaner. By the time I ETS'd, I had all the tricks figured out. Never pulled guard duty in my last 6 months, because I always "made the Man" at guard mount.

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  6. Headspacing an FAL build gets interesting. Not that it is difficult, just that you need a set of different sized parts to adjust it.

    I still have my headspace and timing gauges for Ma Deuce.

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