AR head space?

Muleskinner · Apr 24, 2015

building a new AR "pistol" in 5.56. It's my first "nearly" from scratch build. All the important parts came from the same manufacturer. I have read that adjusting the head space is critical and I have read that as long as the bolt goes completely into battery there is no need...the BCG came pre assembled...do i need to buy a head space gauge and check or are they right about the bolt locking into battery?

SWO Daddy · Apr 24, 2015

I bought a "no go" gauge myself for peace of mind, though AR-15's with bad headspace are rare I'm told.

Muleskinner · Apr 24, 2015

SWO Daddy said:
I bought a "no go" gauge myself for peace of mind, though AR-15's with bad headspace are rare I'm told.

My cousin was an armorer in the Army for 10 years. he says he can count on one hand how many head space issues he saw in that time...My issue is it's ME building the gun...I know my luck...they could give the same exact parts to 24 others to build and theirs would function right out of the box and mine would have the problem...Since my bolt carrier came pre assembled it would mean taking it apart to remove the ejector and probably having issues putting it back together without the tool to safety re-install the ejector..I don't really want to spend the money for something I might only use a few times...

John from MD · Apr 25, 2015

.Since my bolt carrier came pre assembled it would mean taking it apart to remove the ejector and probably having issues putting it back together without the tool to safety re-install the ejector..I don't really want to spend the money for something I might only use a few times...[/QUOTE]

I'm assuming you are not putting on the barrel extension, right? The headspace is set when the barrel extension is installed. As long as you are using milspec parts, there will be no headspace problem.

As far as taking out the ejector. A vise, hammer and a pin punch work just fine for removing and installing. The pin punch will usually retain the ejector as it is going through the bolt so it doesn't fly off. Tie a plastic lunch bag over the bolt if you are worried about the pin flying off.

When you reinstall the pin, start it with a few taps of a hammer and then install the spring and ejector, holding the ejector in the bolt with the back end of the pin punch. You don't have to push it all the way to the bolt face just in until about 1/8 of an inch shows. Tap the pin with the hammer until it is about halfway in and release pressure on the punch to see if the pin has captured the ejector. If so, just tap it down until flush.

Regards,

John

oupa · Apr 25, 2015

John from MD said:
I'm assuming you are not putting on the barrel extension, right? The headspace is set when the barrel extension is installed. As long as you are using milspec parts, there will be no headspace problem.

There's your answer!

Head space should be set with a bare bolt face. An old trick, though not fail-safe is to place two or three layers of vinyl electric tape on the head of a proper sized case and close the bolt. If it closes (locks into battery) you may have a problem. If it doesn't, remove one layer at a time until it does close. SAAMI head space for .223Rem is between 1.4736 & 1.4636, that's 0.010" difference. You'll have to mic your particular brand of tape. THIS IS MERELY AN EXPEDIENT CHECK! It IS NOT a substitute for proper gauges. Proceed with caution!!!!!!! :innocent0

John from MD · Apr 25, 2015

His chamber is 5.56 so .223 headspace gauges won't work for him. There is a way to use them but it is really jury rigging.

John

Mark75H · Apr 25, 2015

John from MD said:
His chamber is 5.56 so .223 headspace gauges won't work for him. There is a way to use them but it is really jury rigging.

John

Hard to find a real 5.56 chamber regardless of what is stamped on the barrel

clandestine · Apr 25, 2015

I have seen lots of AR'S with bad headspace. There's way too many manufacturers and the variable of tolerance stacking to just blindly say your headspace is ok. For what a smith charges to check your headspace you can buy a Go and Field Gauge. You do have to take the bolt apart unless you have military gauges. IMO it's best to buy a .223 Go and a 5.56 Colt Field 2 gauge. This covers the widest range of options.

Every AR shooter should have headspace gauges. Not having them is like having g a car with no instrument panel and no fluid indicators in the engine.

As mentioned by Mark, the throat is what is the main difference between .223 and 5.56 and most 5.56 marked guns do NOT have true 5.56 throats. My students can tell you that, they saw first hand.

IgotAgun · Apr 25, 2015

You don't need to.

Overboost44 · Apr 25, 2015

How about the Hornady kit? If one has several calibers, would this be a less expensive but just as effective substitute?
http://www.midwayusa.com/product/47...hing-set-with-comparator?cm_vc=ProductFinding

John from MD · Apr 25, 2015

Overboost44 said:
How about the Hornady kit? If one has several calibers, would this be a less expensive but just as effective substitute?
http://www.midwayusa.com/product/47...hing-set-with-comparator?cm_vc=ProductFinding

The Hornady kit you are referring to is for adjusting your ammunition to fit your gun. The headspace gauges that we are discussing are used to determine the actual headspace from the chamber datum point to the bolt face when the bolt is in battery.

Regards,

John

oupa · Apr 25, 2015

Good read applicable to the 5.56 vs. .223 question... and a video for those in a hurry and seeking the condensed version.

FlatsFlite · Apr 27, 2015

I bought a barrel and bolt combo from a leading accuracy manufacturer and the head space was way off. They fixed it and apologized profusely.

HT4 · Apr 27, 2015

clandestine said:
I have seen lots of AR'S with bad headspace. There's way too many manufacturers and the variable of tolerance stacking to just blindly say your headspace is ok. For what a smith charges to check your headspace you can buy a Go and Field Gauge. You do have to take the bolt apart unless you have military gauges. IMO it's best to buy a .223 Go and a 5.56 Colt Field 2 gauge. This covers the widest range of options.

Every AR shooter should have headspace gauges. Not having them is like having g a car with no instrument panel and no fluid indicators in the engine.

As mentioned by Mark, the throat is what is the main difference between .223 and 5.56 and most 5.56 marked guns do NOT have true 5.56 throats. My students can tell you that, they saw first hand.

Can you expand on this? I always assumed that 5.56 and .223 had the same headspace specs. Why the .223 Go and the 5.56 Field?

outrider58 · Apr 27, 2015

HT4 said:
Can you expand on this? I always assumed that 5.56 and .223 had the same headspace specs. Why the .223 Go and the 5.56 Field?

^^yes, why is there a difference between .223 and 5.56 gauges? If they start at the shoulder, aren't they "identical" in length? I realize there is a wide variation from one chamber to the next, even within the same manufacturer and that 5.56 chambers tend to be a little bit wider/looser. Shouldn't lengths be necessarily the same?

SWO Daddy · Apr 27, 2015

outrider58 said:
^^yes, why is there a difference between .223 and 5.56 gauges? If they start at the shoulder, aren't they "identical" in length? I realize there is a wide variation from one chamber to the next, even within the same manufacturer and that 5.56 chambers tend to be a little bit wider/looser. Shouldn't lengths be necessarily the same?

Mark75H · Apr 27, 2015

The difference in 5.56 and .223 is almost all in the throat at the bullet. As I understand it, headspace refers to the bolt to case clearance dictated by the chamber.

The barrel throat difference in most simple terms is whether the bullet is engaged in the rifling (.223) or not (5.56). Having the bullet engaged in the rifling in the chamber increases accuacy, but also increases pressure in the cartridge as the bullet leaves. 5.56's extra space gives some expansion room before the bullet is trapped and slowed in the first turn of rifling, a safety factor allowing a hot load, at the expense of a little accuracy.

A lot of "5.56" chambers are modified, but not exactly like a Wylde, to get around infringing on Wylde's patent.

outrider58 · Apr 27, 2015

SWO Daddy said:

Thanks for that, Daddy! I can see the differences but, from what i'm reading, there are sometimes variations in these chamberings even within the same caliber and same manufacturer. What is the next step one takes upon failing a HS check? Recutting? Paper weight?

smdub · Apr 27, 2015

That PTG img doesn't list the important lengths that control headspace, only the diameter differences. So basically meaningless to use to compare headspace data.

If you fail headspace you're going to have to turn the barrel extension in relation to the barrel and repin. This causes complications w/ the existing gas port hole location and the existing extension pin hole. You'd likely need a new extension since it probably doesn't need to turn much (unless they REALLY screwed it up.) Or you could move the pin something like 90deg and drill a second gas port hole but thats totally ghetto

Only if headspacing is too tight you could ream the chamber *IF* its not chromed or nitrided. Though a new barrel extension can fix the problem in either direction.

Edit: Nix that. Forgot about the bbl extension feed ramps. (Doh!) Therefore you have to turn the barrel to headspace it so you're going to end up w/ a misaligned / multiple holes for the gas port. Brain fart on my part...

oupa · Apr 27, 2015

...and if anyone isn't thoughly confused, here's a snippet of advise (warning) from the Brownell's catalog.

"MIXING AND MATCHING BRANDS OF HEADSPCAE GAUGES Don't do it !! That's the short answer, here's why. It boils down to tolerance stacking. Each Headspace Gauge manufacturer works within a range specified by SAAMI. Manufacturer "A" may work at the high end of the range while Manufacturer "B's" gauges are in the middle of the tolerance range. Mixing the two could give an inaccuratereading. By sticking with one brand within a particular caliber, you will eliminate a variable. If you have a Forster, .308 GO gauge, get a Forster, .308 NO GO. Use Clymer with Clymer and Manson Precision with Manson Precision, etc. You can use any brand of headspace gauge with any other brand of chambering reamer. e.g. Forster Headspace Gauges with Clymer Reamers, Manson Precision Gauges with JGS Reamers, etc."

Everything I've ever seen says datum point to head ("head space") is the same for both. It is other dimensions as well as loading specs that creates the differences. "head space" is a specific measurement. It has nothing to do with lead, throat, diameter or anything other than the datum point to head length. Factors affecting pressure are numerous (including head space) given the same primer/powder/bullet combination. As only one example, pressure can be directly correlated to velocity. I have no strain gauge but I have chronographed identical loads (.308Win/7.62NATO) from the same chamber using various brass and even though I expected it, the variations surprised even me. Mil-surp brass is thicker, thus reducing volume / increasing pressure with the same load. Commercial brass differs from brand to brand as well, so that caveat in the reloading manual about sorting your brass has definite purpose! ...and I have stayed at a Holiday Inn! :cool:

AR head space?

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