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Fine Tuning The Headspace

Hi Gus,

Thank you for an incredibly informative post on an often-misunderstood and important topic. I had a headspace issue that I addressed before you wrote this post and I hope that you can further clarify a few points to help me fine-tune the headspace before I go out and break in my new barrel.

I have only one M1A. It is very close in concept to your “Walter Mitty” rifle.
The receiver is a Springfield M1A pre-ban #072,*** (March 1993) & the bolt was made by TRW.
I relieved the receiver’s rail for proper bolt roller clearance and the rifle passes the tilt test perfectly.
The bolt is centered and engages both lugs evenly with excellent contact after lapping.

I had the barrel replaced with a chrome-lined 18” medium weight barrel manufactured by CBI.
The markings on the barrel are: 7790190 7/07 2F CBI.
I had requested a headspace of 1.633” for maximum ammo versatility, but after checking the rifle it was obvious that the headspace was much tighter. I could tell that the gunsmith did not lap the bolt. I stripped the bolt and carefully hand cycled 5 different types of factory 7.62 ammo through it. The bolt did not close completely on most of the rounds and the op rod needed pressure to hop over the bolt roller and snap shut.

Factory M80 & M118 ammo used:
Federal 308 GMM
German MEN M80 (1975)
Lake City M80 (1984, 1985)
Lake City Special Ball M118 (1987)
Lake City M118LR (2005)

The only headspace gauges that I could find were the Forster 308 gauges.
I ordered the Forster 308 GO (1.630”) gauge to check for the minimum headspace for 308
& the 308 NOGO (1.634”) gauge to check for the minimum headspace for 7.62.

My stripped bolt closed perfectly on the 308 GO gauge (rifle squeaky clean & a new unused barrel).
The stripped bolt would not close on the 308 NOGO gauge. No wonder the 7.62 ammo was tight!

If you use .308 Winchester Headspace gages, exclusively, you can still chamber the rifle correctly for 7.62mm as well. You want the bolt to just close without friction on the NO GO gage. That will ensure you have the minimum for 7.62mm and not too much headspace for the .308 at 1.638".
I lapped the bolt and now it just closes completely with very light pressure on the 308 NOGO gauge.
I did this work before you wrote this post. This is my rifles current condition.

*Here is where I am in need of your input:
In light of your previous problems with Forster gauges, I realize that I cannot assume that my headspace is now 1.634”. I think it is pretty close, but the bottom line is does the ammo that I will be using properly fit my rifle or in other words, how should ammo fit in an M1A? I’m trying to adjust the headspace to the minimum required to properly fit factory 7.62 M80 & M118 so I could still be able to use 308 if needed.

When I insert rounds into the chamber and twist the stripped bolt closed by hand, the bolt “clicks” completely shut on the majority of the rounds. On a few of the longer rounds (approx. 5%), the bolt comes within approximately 1/32” short of closing (no “click”, the same fit as the 308 NOGO gauge). This is exactly how much twisting free play that my bolt has in battery with an empty chamber (the op rod does not completely close the bolt with an empty chamber). So at this point, the longest rounds fit perfectly with zero free-play.
I can twist the bolt completely closed with these longer rounds chambered (transitional fit), but the op rod does not provide that last little bit of pressure to twist the bolt completely shut. The op rod is in excellent condition.

MINIMUM HEADSPACE: This is the amount of room necessary to ensure the longest cartridge manufactured to correct specifications will fit and will have enough room to expand to seal the chamber. There has to be enough room for the cartridge case to expand so there isn’t too much pressure on the rifle.
Is my current headspace considered acceptable and safe to shoot or should I increase the headspace a little more? If I increase the headspace to where the bolt just “clicks” shut with the longest round, then the bolt will have approximately 1/32” of twisting free play. Is this enough or should there be a little more fore and aft free play as well?

Thanks again for all of your contributions to this website!
 

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Headspace

Hi Gus,

Thanks for taking the time to read through my post and offering your input. I’m sorry that I don’t live closer to you, but I do believe that I am finally good to go! I will be breaking-in my rifle after Labor Day. It’s been a long, long time coming.

I finally found and just received a Clymer 308 Winchester NOGO gauge. Because of your experiences, I didn’t totally trust the Forster 308 NOGO gauge. It turns out that my Forster gauge is about 0.001” shorter than the Clymer gauge, so my rifles headspace in my above post was actually just under 1.633”. This is probably why the longest of the rounds felt a bit tight.

If you use .308 Winchester Headspace gages, exclusively, you can still chamber the rifle correctly for 7.62mm as well. You want the bolt to just close without friction on the NO GO gage. That will ensure you have the minimum for 7.62mm and not too much headspace for the .308 at 1.638".
I decided to further lap my bolt until my new Clymer 308 NOGO gauge fits as you indicate above. I can now hear the bolt lug “click” against the receiver and when the bolt is closed there is zero fore and aft free play.

All of the factory 308, M80 & M118 rounds that I have now chamber perfectly!
Your advice regarding the 308 NOGO gauge is perfect for maximum ammo versatility.

Thank You!
 
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