Checking Headspace on AR

[quig]

Ok, after a little incident that I had last week, I decided to be safe and check the Headspace on my AR.

I am familiar with the AR, for I have done a couple of builds, but never bothered with checking the headspace.

So I purchased the Colt, headspace tool from Brownells. There is no video, but just some instructions.

So far, I haven't seen too much on the Internet that has helped. Mine isn't the Go-No Go gauge.

So according to the instructions, I removed my extractor and placed in the headspace gauge. With the upper only, the BCG will not fall all the way into the battery by gravity. If I give the BCG a slight push, it will go all the way forward and back out when I release it.

I don't know if any of this makes sense, but I am just throwing it out there to see if any one has used this gauge.

Thanks

[swifty]

Ok, after a little incident that I had last week, I decided to be safe and check the Headspace on my AR.

Mine isn't the Go-No Go gauge.

What gauge did you get? What was the incident that makes you want to check headspace?

"Close" means the locking lugs rotate fully into the locked position.

The bolt should close fully on the "GO" gauge.

The bolt may close on the "NO GO" gauge.

The bolt absolutely should not close on the "FIELD" gauge.

[nicoroshi]

It is possible to force a bolt closed on a 'No-Go' gauge even when the chamber is in spec.
Would be better checked with a field gauge if you truly want to know it won't close.
The difference between the go, and no-go gauges for .223 is .003" (civilian). The military leaves a bit more leeway (same as civilian +.0006").
The field gauge is even larger, and the top end of what is considered acceptable.
Here's something that will help understand the differences:

http://www.ar15barrels.com/data/headspace.pdf

Very close to the same for 7.62x39 between the GO, and Field gauges.
Here's how I check (shown on an AK but same idea).

Caveman headspace check

Trick is light finger pressure only when pushing forward.

Hope this is helpful,
Nico

[a308garand]

The Brownells video correctly shows checking headspace with the bolt stripped of the ejector. You can also remove the extractor and do the check with a stripped bolt.

Ensure the chamber is very clean, scrubbed with a chamber brush to remove any fouling that can throw off your readings.

The gauge is inserted into the barrel chamber. The bolt group is inserted into the upper receiver. Using finger pressure only, slide the bolt group forward and look to see if the bolt goes into battery.

If the bolt does not rotate into the receiver extension (go into battery) you will see the rear of the bolt carrier sticking out of the upper receiver by at least 1/8 inch or more.

If the bolt rotates into the receiver extension (goes into battery), the rear of the bolt carrier will be flush with the rear of the upper receiver.

Depending on the gauge you are using, the bolt may or may not close.

GO = bolt closes into battery.
No Go = May slightly close, but not fully close into battery.
Field Reject= Bolt should not close into battery.

If bolt does not close on "Go" gauge, chamber is short for that bolt. Either try another bolt or cut the non-chromed chamber slightly deeper.

If bolt closes on "Field Reject" gauge, chamber is too long for that bolt. Either try another bolt or barrel.

So which gauge do you have?

edit: You other guys beat me to the punch.

[quig]

Ahhhhh,

This is just my lack of understanding of the headspace gauges. Mine is the field gauge. And, in fact, my bolt will not close on it, so I should be good.

Here is the incident to speak of:

AR Kaboom- Just a little

Thanks Guys!

[swifty]

"Field" is the gauge you want. Sounds as if your headspace is fine.

I read through the other thread and I'll toss out another theory. Some general information first.~ Headspace is an internal dimension of the chamber that measures the distance from the face of the bolt to the datum line of the shoulder. ~ Cases are made considerably heavier through the wall area of the portion of the case that protrudes from the chamber.

It is quite possible that the length of the barrel at the breech is a tad short. If this happens you may experience an "Incipient Case Head Separation" even though headspace is correct and the cartridge is correct.

[quig]

Checking Headspace on AR

Thanks Swifty, but what is the solution?

Do I get this measured? Is it just this ammo? What should be my next "move"?

[kendog4570]

It is quite possible that the length of the barrel at the breech is a tad short. If this happens you may experience an "Incipient Case Head Separation" even though headspace is correct and the cartridge is correct.

With a 60 degree feed cone, proper clearance of .005"-.007" from the front of the bolt to rear of barrel for adequately supported case. Any less and dirt and debris can jam the bolt, any more and the thin part of the case walls are exposed and unsupported. It will fuction, but the safety factor designed into the mechanism is compromised.

Also, the lug seat on the AR is square to the centerline of the bore, so the bolt will either close or not close on any given gage. There is very little or no camming action. You can force it on a close fit, but that is not the proper use of the gage.

[swifty]

...but what is the solution?

Do I get this measured? Is it just this ammo? What should be my next "move"?

IF the barrel is too short...replacement is the only good option.

The barrel can be measured, but it is not easy to do.

Ammunition is a contributing factor, but not the cause of the problem.

The barrel manufacturer should be able to compare the barrel to their spec sheet.

<<<<<>>>>>

This is where having the damaged case would really come in handy, for inspection and diagnosis. At this point you could section a fired case and look for too thin walls in the suspect area. Currently, the cases may just barely be containing the pressures.

The only examples of this cause of case damage I have seen resulted in the case looking like it had been slit. Very clean precise slit with some of the brass laid back flat against the breech end of the barrel.