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Old November 21, 2012, 00:49   #1
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M1A Head space help

Hi all, Needing some help, I'm back working on my home brew Socom16. Here is the run down on the build. I'm using a Norinco receiver, lrb bolt and a SA barrel. have the barrel timed and I lapped the bolt and now have great lug contact. Now I need to set head space the bolt will not close on my 308 gage. It starts to close but just will not go. really don't want to lap the bolt any more. The barrel is a SA factory take of so I don't think it's crome lined. I was going to rent a pull through reamer but what I need to know do I cut it for my 308 gage or do I go deeper to shoot surplus? Any input would be great.

Thanks Dave
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Old November 21, 2012, 03:12   #2
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I am not sure but I don't think you want it to close on a .308 gauge, I think you may be exactly where you want to be...let me get with my Dad and I'll get back to you...he is up to speed on M1A headspacing.
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Old November 21, 2012, 11:03   #3
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First of all, WHICH .308 gauge are you talking about? GO, NO-GO, or FIELD? Closing on the GO is required but not desirable for the other two. Closing on the NO-GO and using only mil-surp ammo and brass is not a bad thing since most .308 NO-GO gauges are smaller than the military spec 7.62 gauges.

I have set my headspace on all of my military surplus rifles to just close on the GO gauge. I have fired many rounds of just about every type of mil-surp ammo through them and haven't had any problems with chambering or pressure. And because I am running the tighter chamber, I have not had any problems when using commercial .308 ammo either.
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Old November 21, 2012, 11:57   #4
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First of all, I'm not an armorer, so this is only my opinion based on barreling a few Garands and M1As for myself. On the Garand types, you definitely do not want too short of headspace. There is currently a thread going over on the m14 forums about this. Too tight headspace has been linked to KB's. Personally, I think it is due more to the quality of the ammo.

With the Garand/M14 types you are kinda lucky, because when checking the headspace by hand, with no op rod, you can pretty much see when you bolt is nearing fully locked by the position of the bolt roller and right lug.

The pull through reamers are meant to be used with the pressure of the bolt providing the push for cutting. If you keep it clean and well oiled as you cut, when the bolt is all the way closed, your chamber is done. You should be at a point between the go and nogo limits. Some better pull throughs have bushings that allow you to adjust this. The cheaper don't. Most barrels out there are short chambered and you will only need to cut a few thou at the shoulder to get your headspace.

I bought a set of match HS gages way back when (Forster) that are made in .001 increments. I had them laid out by one of the inspectors at work and all were within .0002" of what the nominal was supposed to be for that size. So I trust them to give fairly accurate measurements. The headspace with my pull through in 308 measures between 1.632 and 1.633. YMMV.
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Old November 23, 2012, 18:23   #5
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USGI "field reject" FSN# 4933-647-3698 (sized 1.6455")
USGI "no go" FSN# 4933-916-9275 (sized 1.6375")
USGI "go" FSN#4933-916-9271 (sized 1.6355")
*referenced from USGI manual #TM 9-1005-223-34

Hope this clears the muddy waters.
Cheers, YV

Note; the USGI 'field reject' is only used with a special test bolt, not with the rifles bolt.
It's only used to test for stretched recievers, so they can be removed from service, and has no bearing on anything else.
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