M1A and Odd Headspace Question

[Welding Rod]

My M1A's bolt won't quite close on a 1.638" headspace gauge (Forster .308 Win field gauge). The gauge goes easily into the chamber.

I used my RCBS Precision Mic (.308 Win) to measure both the headspace gauge and the brass that I fired in the rifle, with the gas valve shut.

The fired brass measures .006" longer than the headspace gauge that the bolt won't quite close on.

Couple of details: Like new rifle probably has less than 500 rounds through it. New Black Hills Match ammo was used when shooting with the gas valve shut. The bolt was stripped, and all parts, chamber, and Precision Mic thoroughly cleaned before measuring. Headspace gauge is brand new and there are no burrs on the brass.

With the gas valve open the brass stretches an addional .003". Various loads stretch about .013" to .018" total on one firing depending on if the gas valve is open or closed.

Any ideas how my fired brass is coming out substantially longer than the chamber when I am shooting with the gas valve shut and hand cycling the action?

 

[TonyAngel]

OK, I guess I might be putting my foot in my mouth because I haven't messed with M1As much, but I believe that the headspacing is done or measured from the shoulder, not the rim of the neck.

My guess is that you're getting slightly more stretch with the gas port closed because there is slightly more pressure when the gas port is shut off.

 

[FlyinBryan]

possibly an anomily with the finishing reamer shoulder that was used to do final headspacing when the barrel was installed?

 

[bigedp51]

My M1A's bolt won't quite close on a 1.638" headspace gauge. (Forster .308 Win field gauge).

Question, WHY would you want your bolt to close on a FIELD headspace Gage, that is larger than the NO-GO gage?

Does the bolt close on the NO-GO gage?

The shoulder angle/datum point of your fired cases could be different than your "FIELD" gage.

All you need to do is push the shoulder of the case back .001 to .002 shorter than your actual headspace.

I use the GO gage as zero reference point with my RCBS Precision Mic

 

[Welding Rod]

I have no desire for my bolt to close on a Field gauge. But since the brass was stretching so much I bought one, expecting to find that the rifle had excessive head space. I thought the field gauge would probably fall right in judging by the headspace measurement of the fired brass. I even bought a 7.62 MAX gauge thinking it possible that it may also go in. However the Field gauge shows that excessive headspace in the chamber is not the case.

I do not have a No-Go gauge.

Hmm. Thanks for thoughts. My best guess is that the shoulder area in my chamber must not be flat, allowing the shoulder of the case to blow out into a convex shape... I will investigate further.

 

[Slamfire]

Question, WHY would you want your bolt to close on a FIELD headspace Gage, that is larger than the NO-GO gage?

Does the bolt close on the NO-GO gage?

A very good question indeed.

7.62x51mm NATO Go 1.6355"
7.62x51mm NATO Field Reject 1.6455"
.308 Winchester Go 1.630"
.308 Winchester No Go 1.634"
.308 Winchester Field Reject 1.638"
7.62x51mm NATO minimum cartridge headspace 1.630
7.62x51mm NATO maximum cartridge headspace 1.633
7.62x51mm NATO average cartridge headspace 1.6315
.308 Winchester minimum cartridge headspace 1.627
.308 Winchester maximum cartridge headspace 1.633
.308 Winchester average cartridge headspace 1.630

http://carnival.saysuncle.com/002453.html

The fired brass measures .006" longer than the headspace gauge that the bolt won't quite close on.

With the gas valve open the brass stretches an addional .003". Various loads stretch about .013" to .018" total on one firing depending on if the gas valve is open or closed.

Any ideas how my fired brass is coming out substantially longer than the chamber when I am shooting with the gas valve shut and hand cycling the action?

See that inset in the pressure curve?. Your rifle is acting as it was designed. It unlocks somewhere below 650 psia (about) which is the rupture strength of your brass. This is called the residual blow back effect, look it up in your Machine Guns, Vol IV by Chin. This period increases the amount of time useable gas pressure is available to operate the mechanism.

It also stretches brass all to heck. But then, an Army reload is a box magazine of new LC ammunition. The Army does not reuse/reload once fired brass, or any fired brass.

The scenario is basically this. Firing pin hits cartridge and drives cartridge forward till the shoulder hits the chamber front. Primer backs out. Case neck and shoulder expand locking the front of the case to the chamber. Pressure builds and the case head locks in. Around 25,000 pounds the sidewalls stretch and the case head moves back to the bolt face shoving the primer back in the pocket. Somewhere between 1.3 and 2.3 milliseconds unlock occurs and the bolt starts moving back. The front of the case, still glued to the chamber shoulder is forcefully yanked off the chamber walls, the pulling of which causes more sidewall stretch. Before the bolt hits the receiver heel, the cartridge is ejected from the receiver.

 

[bigedp51]

SlamFire1

Do you actually believe all 7.62 NATO rifles have the "same" headspace requirments?

British L-42 sniper rifle
1.628 minimum (.002 smaller than U.S. military or SAAMI)
1.635 maximum

British L1A1
1.6325 minimum
1.6430 maximum

You might want to check your numbers for the "M14"? Your giving out five diffrent headspace numbers for the M14 rifle.

As you can see below information on the internet varies, it makes me wonder what the military manuals have to say on the subject.

4. Different headspace dimensions are defined by different manufacturers and organizations.
An excellent example is the 7.62mm/.308 cartridge:
CARTRIDGE MIN MAX
SAAMI 1.627 1.634
GOVERNMENT 1.630 1.633

CHAMBER MIN MAX
SAAMI 1.630 1.640
GOVERNMENT M-14 1.6355 1.6385
GOVERNMENT M-14 Match 1.631 1.633


http://www.armalite.com/images/Tech%20Notes%5CTech%20Note%2069%20Headspace%20080722.pdf

 

[Jason_G]

I believe SAI M1As (newer production at the very least) come from the factory with headspacing between 1.631 and 1.632, if that's any help.

Jason

 

[W L Johnson]

Am I reading that chart right? It looks like it says 7.62x51 pressure peak is 50 kpsi when it should say 50 kcup, big difference in pressure. 7.62x51 is not 50,000 PSI it's 50,000 CUP. This is where the misconception of a pressure difference between 7.62x51 and 308 comes from.

 

[Onmilo]

All the pretty colors,,,,

Are you shooting .308 Winchester in your rifle?
You probably have a rifle that is chambered to 7.62 Nato standards so you are experiencing case stretch of the commercial dimensioned cases.

This is why I recommend you do not reload commercial cases fired in 7.62 chambers more than twice as the excessive case stretch will cause case failure in very short order.

As far as safety when firing new ammunition, fuggitaboutit, no worries, commercial cases are safe to fire in a NATO chambered rifle, just don't expect to get too many reloads out of them. HTH

 

[bigedp51]

Welding Rod

Onmilo, just made a very good point about commercial ammo in a military chamber and case stretching in the web area. The .308 case below stretched .009 when fired in a surplus 7.62 Enfield rifle and I chucked them in the trash and didn't even think of reloading them and they were only once fired.

NOTE: You also run a greater risk of warped banana shaped cases when firing commercial cases in a military chamber.

 

[Welding Rod]

Slamfire - Thanks for info. I don't know if you caught it or not but I am confused about how the brass is stretching so much when the gun is being fired with the gas valve shut and not cycling in semiauto fasion.

Right or wrong, Forster markets their "7.62 Max Chamber guage" at 1.6455" not 1.633". Why they don't call this particular gauge a field gauge I don't know: http://www.forsterproducts.com/catalog.asp?prodid=700599&showprevnext=1

Flex in the bolt/reciever sounds like a logical contributer to the condition... a poster in another forum suggested that. Also, maybe a shoulder in the chamber that is not flat but slightly concave.

The tag on the gun from SA said headspace was 1.631 when it left the factory.

I shoot 7.62 commercial, 7.62 military, and .308 commercial loads in the gun, all with similar results.

And just to be clarify again, when I talk about the stretch in the brass, I am refering to stretch measured in the headspace dimension of the brass, not the overall case length.

BTW, the gun seems quite accurate for a Standard M1A. It will shoot 1.5 MOA 5 shot groups with match grade loads and iron sights.

 

[bigedp51]

Welding Rod

The RCBS Precision Mic is not the best of gages, I know I have one in .308, it is used for ball park measurements for setting up your resizing dies. The angle of the shoulder in the Precision Mic for reading the datum line could be off. Or the shoulder of your chamber could be off at the datum point, or both could be off.

On my RCBS Precision Mic it reads "minus" .003 with a .308 GO gage in the gage, the GO gage is 1.630 and the Precision Mic is reading .003 less than this. Simply put the Precision Mic is out of calibration, I emailed RCBS and asked if they could recalibrate it and the reply was forget about the zero reading and just count the lines to get the measurements.

Question, using the Precision Mic how long are the cases before they are fired in your rifle, and how long are they after they are fired. This would be total case stretch and give you another base line measurement.

 

[243winxb]

the Precision Mic is out of calibration,

Question, using the Precision Mic how long are the cases before they are fired in your rifle, and how long are they after they are fired. This would be total case stretch and give you another base line measurement.

He said > .

Various loads stretch about .013" to .018" total on one firing depending on if the gas valve is open or closed

 

[243winxb]

Most dies/shell holder will automatically set a safe working cartridge headspace.

SAAMI 308 win. Maximum chamber 1.640" Minimum cartridge measurement 1.627" = .013" difference or possible case stretch on firing of factory ammo or new brass. http://www.saami.org/PubResources/CC_Drawings/Rifle/308%20Winchester.pdf

 

[bigedp51]

243winxb

What I would do is seat a fired spent primer in a new unfired case with just my fingers, and then chamber this test round with the bolt fully closed. The amount the primer is protruding after the case is removed from the chamber would be his head clearance or the "air space" between the bolt face and the rear of the case.

If the head clearance is only a few thousandths then either the case is stretching badly in the web area or the shoulder is moving forward or both. I do not own a rifle of this type but how is lockup controled and could the bolt be opening too soon causing the case to grow under chamber pressure?

 

[Welding Rod]

The cases show -.001" to - .005" on my Precision Mic before firing. Most are -.001" to -.003" on the Mic. The stretch in the head space dimension I indicated (.013" to .018") is the difference from the "before shooting measurement" to the "after shooting measurement".

Biged - Yes port pressure plays a role. When handloading the brass stretches an addional .003" or so with slower powders like Varget and in range stated above with faster powders like H4895 or IMR 3031.

 

[Slamfire]

Slamfire - Thanks for info. I don't know if you caught it or not but I am confused about how the brass is stretching so much when the gun is being fired with the gas valve shut and not cycling in semiauto fasion.

This is a problem, I read enough to assume the question. Sorry, do it all the time, and get it wrong all the time.



As for brass being longer if the rifle does not cycle, I think you have a gage issue.

Do you have a Wilson type gage rolling around? I have dropped chamber head space gages into my Wilson gage and lo and behold, they correspond exactly with the “go” and “no go” of the gage. So the things are made to a calibrated standard, they are exact, whereas the Precision mike you use is a relative gage.

As to what is causing this, since I don’t have a precision mike, I don’t know. Maybe the engaging surfaces and the shoulder angles are just not lining up.

Or maybe you have one of those weird chambers that are bigger on the inside than on the outside. Time dimensional space aberrations or something, black holes and alien technology.

 

[Onmilo]

Since you mention Black Hills Match ammo which is usually loaded in 7.62X51 cases and you mention the cases are stretching when ejected from the rifle manually, I can oly guess the barrel throat leed is a bit tight and the case is dragging and stretching as you manually extract it from the chamber.
Because extraction occcurs in milliseconds during automatic cycle function these case has less time to seize at the barrel throat.