Headspacing Question

How do you know if your headspacing is off on an AK? The reason I ask is I just got a M92 Krinkov. took it out. I had two different types of Wolf ammo. Black box and tan camouflage box. I shot up the black with no probs. then when after a half mag of the tan box stuff I noticed it sounded different. at the end of the mag i noticed one of the primers had backed all the way out. I havent shot it again. thanks

You determine if the headspace is correct by using headspace gauges. There are three, commonly called a Go gauge, a No Go gauge and a Field gauge.
You can determine if excessive headspace is causing your problem using a field gauge.

ak-builder.com sells headspace gauges

You're not going to believe this, but headspace problems are quite rare. After experience with uncountable rifles over a lifetime I have only seen it two or three times. Anyway all "headspace" is is the distance between the back of the chambered cartridge and the boltface. On a stamped reciever AK the headspace is set when the bbl is "notched" or drilled at the same time with the trunion. The bbl is pressed in, headspace VERY precisely set by a very experienced person, then bbl and trunion drilled as one unit. This is the single most critical operation in the manufacture of an AK! The only ways an AK can "lose headspace" is if the bbl would somehow come loose,Thus changing the fit of the bbl end to the breechface, or if the reciever itself would be "soft" and stretch. I suppose its possible for a bolt and bolt carrier to wear out, but I've never seen it, even after lots of full auto fire...genenrally the rest of the AK wears out first. Anyway if it is headspace, its gonna cost some $$$$ to repair, if possible...the reciever might be unsalvagable if done wrong. Re-doing a bbl stamped reciever is way more expensive than just getting a new reciever. Headspace guages are made so that there is about .003 to .004 difference between the GO and NO GO. If the rifle bolt will close on the GO guage, it means that the rifle is safe for SAAMI spec'd factory ammo, if the bolt will close on a NO GO guage then the chamber is too long...and too dangerous to fire. Backed out primers ain't good, but if it was only one cartridge...or even a whole box it might be the ammo...its possible, Russian ammo is notorious for uneven quality...Finland considers it "unserviceable". The specs for Russian ammo is all over the place, and every European reloading manual is different. No SAAMI specs there! Just my .02

This is correct. However, not everyone in the United States who is setting headspace is "... a very experienced person...". In fact some are completely inexperienced and go by the name of Bubba.
Many of the M92 kits came in with virgin barrels and it is possible that the OP's was built incorrectly.

Yea, and I was trying to be polite. Building an AK isn't rocket science, Pakastani Tribesmen do it everyday...but they have experience...I read that even Mark Krebs really valued what he learned by visiting Izmash and observing how they bench built AKs...and that he purchased some of the correct tooling to make builds easier. Armory USA also brought origonal guages and tooling from Bulgaria that did not exsist here, and it helped. Lots of great builds have been done at home, by talented people but you don't hear too much about the "oops" models. I learn everything the hard way...but its a hobby.

And to complicate the matter further, it is not just whether it will close on a particular gauge, but how many kg pressure to close - and that spec is different on the same gauge in different weapons with different locking systems - SKSs having less locking pressure specified than AKs - and there being a completely different gauge for a 7.62x54 SG43 than DP28.

Some have stated that the SAAMI and arsenal gauges are different. While arsenal gauges are different from one country to another, it is where the datum point - the "front" spot on the taper contact is, and while the gauges are different, they are the same data point within the three gauges from the same manufacturer.

In testing the pressure for close, I found my Chinese Arsenal, East German Arsenal, and SAAMI gauges to all be the same. The OAL of the gauge isn't important, only the distance from that manufacturer's datum line to the base being the same within the three gauges.

So would the bolt from, say an Arsenal SGL31 be interchangeable with another Arsenal SGL31?

Thanks

Interchanging bolts.

Yes they are "interchangeable"

BUT (there is always a "but")

If the gun was headspaced with a new bolt, and you swap for a worn bolt, the headspace could end up long.

If it was headspaced with a worn bolt, and you replace with a new bolt, the headspace could be tight.

remember the difference between a GO and NOGO is .004" - a human hair is typically around .003"

So it is prudent to always check, but if I'm repelling zombies, getting the gun back into any form of operation beats having the zombie eat my brains.

That being said, the nature of the tapered cartridge and the mass of the bolt carrier and large locking lugs makes for a pretty forgiving system. So while I don't want to imply that headspace doesn't matter, there is a difference between what is ideal in the shop when installing a new barrel and what is practical on the battlefield to get a gun back into operation. When installing a barrel, I strive to obtain minimum headspace specifications. I believe a gun will continue to function just fine with excessive headspace.

The field gauge dates from a time of more primitive metallurgical technology and "hand fitted" parts. It was a method of checking a rifle in field service. I suspect there is a whole lot more variation in cartridge headspace than you will ever find in chamber headspace.

I do not consider there to be much risk in shooting a gun that closes on a NOGO. I'd use a gun that closes on a field only as a last resort.

Investing $40 on a go and nogo gauge is cheap insurance when choosing which rifle to buy.

Thanks. I've been wondering about this for some time. So if I want an extra bolt I will have to have it hand fitted?

If so do you do this kind of work.

Thanks again.

We'll have to do this like Moses and the Israelites.

––––––––––––––––-

I send you Prophets and I send you preachers. Sages in rages and ages of teachers, but still thy seek to bargain with thy Lord your God.

Thy wisdom leaketh like a cracked pot. Ask not of the Holy Hand Grenade of Antioch for its wisdom is beyond thy grasp.

No longer shall thy Lord your God bear witness to equivocation and stiff-neckedness.

Bargain not with thy maker, for a veil of ignorance shroudeth thy head.

Heed me for verily! I say unto thee.

Thou shalt purchase a GO and a NOGO gauge.

Though shalt insert the GO gauge into the flower of thy boom stick and closeth thy bolt.

Thy bolt must close

Thou shalt removeth thy GO gauge and replace with thy NOGO gauge

Thy bolt must not close. If your bolt closeth not, praise be unto thy God for he is is merciful.

Should thy bolt close on the NOGO gauge, it's spirit is tarnished, like the sickness of the vine, but all is not lost.

Purchase for thyself and thy house the gauge marked thus "FIELD". Make an offering of 2 doves to thy Lord your GOD.

Insert thy FIELD gauge into the flower of thy boomstick.

Should the bolt closeth not, be praiseful unto thy God, but watcheth thy boomstick and test it when the moon grows full, for it may fall under the guile of darkness.

Should thy bolt closeth on thy gauge marketh "FIELD", it is full of wickedness and iniquity. Like the vine that beareth bad fruit, it must be rippeth from the earth. Seek ye another bolt.

Got it.

Gunplummer–– beautiful!

Thou hath the word of a wise man.

Perhaps my inquity and stiff-neckedness is great, but if I understand you correctly, what is only required for safe operation of the gun is the GO gauge (for which the bolt shall close) and the FIELD gauge (for which the bolt shall not close). If these conditions are met, then the rifle is safe to fire.

Where can I purchase these gauges at a reasonable price for an AK-74?

7.62x39 - brownells probably has both forster and clymer. I bought both to compare to my german and chinese gauges. 5.45 I got from Pacific Tool and Gauge.

Let me ask you this question. When I first got my AK-74, it would not cycle properly by hand. Once the bolt closed, I could not get it open without a great amount of force. When I took it to the local gunsmith, he checked the headspacing and said it was fine. The bullet wasn't seating far enough in the chamber however. He said this was due to a burr in the chamber when the barrel was pressed in and so he sanded the chamber lightly and since then it has been fine.

Do you think his diagnosis was correct? Should I be worried any further? Will excessive sanding of the chamber make the headspacing too loose?

I really can't say without verifying that there was indeed a bur in the chamber. I have noticed in some sloppy assembly, the builder uses impact on the chamber face and I suppose it is possible, particularly around the the extractor cut, to swage a little bit of metal into the chamber proper.

That being said, the most common cause of tight unlocking that I have experienced is a sloppy factory cut on the unlocking camming surfaces of the bolt, or a bur on the related surfaces of the trunion. Looking at the bevel between the the vertical and horizontal surfaces of the bolt, you will see a radius where that bevel blends with the bolt body. I often see these incomplete - maybe the corner of a cutting tool chipped leaving it of insufficient depth. I just take the edge of a Dremel cut-off wheel and lightly extend that angle.

The sure test - is to sandblast the bolt to bare metal, paint with dykem type dye, and work it - see where the impedance is.

As to sanding the chamber - on a chrome lined barrel, not a good idea. On a non-chrome lined barrel, assuming he only touched the one spot that he claims was bad and didn't affect any other dimensions, I guess it would be okay.

Copyright © 1996 - 2013 :: AR15.Com LLC :: All Rights Reserved.
Any use of this content without express written consent is prohibited.