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MGySgt USMC (ret)
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Discussion Starter · #21 ·
Art,

You didn't mention it, but I assume you have a tapered shoulder on the snap caps that corresponds to the shoulder on the cartridge case just before the neck? Even if not, you can still run into problems with length if you have no shoulder on the snap cap. Here's why.

The Datum line where the forward point of headspace is measured is different between Military and SAAMI chambers and headspace gages even though there is no difference between the .30-06 in Military or Commercial. I stole this link from another post to show you what we mean by the different Datum Lines. In the "Long Chamber" drawings of the link, this illustrates the difference between 7.62mm and .308. The lines going through the shoulder areas show different Datum Points (areas along the shoulder) where headspace is checked.

http://docs.google.com/Doc?docid=0AS0jPehGQnwsZHBxY3Z4al8zNGhzc2M3c2Zn&hl=en

Now, I only VERY rarely have used SAAMI spec. gages in .30-06 as I have a complete set of in spec Military Gages. BUT SAAMI gages read much longer than Military gages for each measurement of GO, NO GO and Field Reject. This means the SAAMI gages have a different angle that goes up against the wall of the angled shoulder in the barrel. That also means the angle of the shoulder of the SAAMI gages are less than the angle on the Military Gages - otherwise the SAAMI gages would not go in as deep and read as long as they do.

I'm not sure if this is clear, but if not, please ask more questions.

We REALLY see the difference in the shoulder angles of Headspace gages in .308 gages vs Military Gages in 7.62mm with the SAAMI gages also having less of an angle.

What I'm pretty sure you are running into is the shoulder on your snap caps don't have enough angle (meaning the angle isn't tapered enough) and can't slip far enough into the chamber on .308 (or .30-06) in Military Chambers. If you reduce the angle of the shoulder on your snap caps and maybe extend the shoulder back towards the rear of the cartridge case, the snap caps should fit fine in both Military and SAAMI spec chambers. IOW, it would be best to fit them to the SAAMI spec and then a .308 snap cap will work well in a 7.62mm chamber as well.
 

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Eye Master
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Thanks for the detail. No doubt this difference between military and SAAMI is adding to the confusion, but I don't know if that is the problem. FOr one, your link suggested military chambers might be longer than SAAMI, so if I make a SAAMI snap cap, it ought to fit easily in a Garand chamber - but it does not.

I will go back and verify my shoulder angles, but I think they were OK. I could see that it was the edge where the case body joined the shoulder that was interfering, so if I keep the same neck, and just incline the shoulder back slightly, it might help, though again, this does not explain why the SAAMI chamber and the SAAMI case have an interference, just because when you look at the headspace numbers, the case is bigger than the chamber. Too strange!

Art
 

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Art,

You are correct about the 'overlap' of a MAX case and a MIN chamber - the same also occurs with 308Win.
I don't understand why there is this overlap in HS dimensions - maybe because of mfg tolerances ?
Or, maybe there is some 'history' of the dimensions that were previously used that carried-over into the current specs?
See here for info about 308Win/7.62Nato -
http://www.armalite.com/images/Tech Notes\Tech Note 69 Headspace 080722.pdf

I suggest that the size of the snap-cap 'cartridge' should be towards the MIN case dimensions.

For a snap-cap I've used a FL-sized case with a piece of hard neoprene cut from a faucet washer in the primer pocket...

Jay Kosta
Endwell NY
 

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Eye Master
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Jay,

1. Thanks you for that link. It is an excellent find, and confirms what I had seen.
2. WTF? THey come right out and say there is an interference inthe stack-up of a long case and a short chamber, and come right out and say that this can cause the bolt to not close ..... but then continue on, as if this is no big deal?

The only thing I can think is that SAAMI is taking the safety perspective, and the size range they are giving will produce a safet cartridge. They are not looking at good function - that is up to the manufacturer. If you look at the SAAMI specs, it suggests there is about a 0.010" range of lengths that will work. THe Armalite article suggested that an acceptable range for function was closer to 0.004" for a rifle, 0.003" for a machine gun, and 0.002" for a NM rifle.

My solution was to shave about 0.006" off the snap cap body length. I'm awaiting samples, but hope this resolves the issue.

In summary, it seems to me that a lot of this confusion on headspace is exactly what is going on here - different people are trying to do different things with these numbers:

SAAMI is looking at safety, and trying to account for wear.
Mil-Spec is looking primarily at new gun production, and possibly accounting for less frequent cleaning.
Manufacturers are looking at targets and tolerances that will work.

Add to this that we are discussing 60 year old rifles, gages and jigs that were built before ISO 9000 and calibrated standards, and that people misunderstand the intentions of numbers, and mis-apply them, it makes for a good confusion soup.

One would think that with today's CNC equipment, and calibrated standards, SAAMI could come out with modern target dimensions that make a little more sense. After all, these days a +/- 0.005" tolerance on a machined part is very, very, sloppy.

Thanks again for your help.

Art
 

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Eye Master
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Interesting, I got automatically censored. When I typed a W, followed by a T, followed by an F, and posted it, the system automatically substituted "what the hey".

I guess the initials for a bad word are now considered a bad word... hey, that's a BAD LETTER. as in "I'm sorry Mrs. Johnson, we had to send little Johnny home from school today, because he used a bad letter".
 

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MGySgt USMC (ret)
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Discussion Starter · #26 ·
Art,

I may not have gotten my point across correctly. Military Chambers are allowed to be longer than SAAMI chambers and that's why I recommended you size the snap caps to SAAMI specs for both .308 win and .30-06. The SAAMI specs call for shorter chamber lengths, so if you make the snap caps so they go in easily in SAAMI chambers, there should be no problems with the length to fit military chambers.
This includes a sharper taper for the shoulders of the gages.
 

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Eye Master
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Gus,

I made them to the max SAAMI dimension (ie the dimension on the drawing, which was +0/-0.010), and they did not fit in my Garand.

I have now shaved 0.005 off the shoulder, and they fit.

My shoulder angles were 17.5 degrees, which is the SAAMI spec.

As I mentioned in my previous post, I suspect SAAMI drawings are max safe dimensions, which does not necessarily mean these dimensions are optimal.

Art
 

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I'm more of a visual learner, so it wouldn't surprise me if the answer should already be obvious, but I figure what the heck.

I picked up my first M1A a few days ago, an SAI Loaded. The headspace tag that came with it read 1.631". After reading through your posts a couple of times, my interpretation is as follows:

-It's good to go for .308 spec ammo, being close to minimum headspace.
-It'd be too tight to safely use 7.62 spec ammo, being .002" or so short.

Am I correct in this? That I should not purchase any surplus 7.62 due to it being A) Hotter than .308, and B) The headspace being tight, or perhaps tighter than acceptable for 7.62 specs?

Thanks, Gus, for providing everyone, M14 vet and newcomer alike, such detailed information.
 

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MGySgt USMC (ret)
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Discussion Starter · #29 ·
-It's good to go for .308 spec ammo, being close to minimum headspace.

That's correct.

-It'd be too tight to safely use 7.62 spec ammo, being .002" or so short.

This may get a bit confusing, so please ask questions later if you don't fully understand.

The White Box NM 7.62mm ammunition was used in these kinds of chambers for NM shooting. This ammunition ran 200 - 300 FPS slower than regular 7.62mm ball ammo and thus the pressures weren't as high.

The 7.62mm ball or other 7.62mm ammo the U.S. government accepted could be safely fired in this chamber as the cartridge cases were held to tighter specs. and were stronger than normal .308 brass. BUT there is a Caveat to that. The U.S. government did not surplus ammo just because they had too much of it. They surplused it when it wasn't made correctly, had the wrong pressure curve, bad primers, etc., etc., etc. The average person has no way of knowing if the U.S. government surplused ammo due to rejection it didn't meet spec. or safety issues. Personally, I would probably fire any U.S. made 7.62mm ammo (except pressure test loads of course) and keep an eye on the brass or anything unusual. This to check if the ammo was surplused due to rejection or safety issues. If the U.S. government didn't want it, I don't want it in my rifle.

GOOD Nato spec 7.62mm also falls into the category I would at least try it to see if there are any problems on why it was surplused. But remember, countries also surplus ammo that is too old, wasn't stored correctly and you may have problems with the rounds going off. That's not really a safety issue, but could show up as some to many failure to fires.

Some 7.62mm ammo that is sold as "surplus" is not Nato spec. I would NOT fire that in this rifle's chamber as the cartridge cases could be too long or the pressure curve wrong or a number of other things.
 

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Gotcha, makes sense. I'd been looking at some Portugese surplus, but the combination of it being 20-30 years old, having read it's 'hot', and my rifle being chambered tight, probably a good idea I avoid it, sounds like.

Perhaps some Brown Bear is in my future unless I take the reloading plunge. =)
 

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MGySgt USMC (ret)
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Discussion Starter · #31 ·
Though a bit hot, I've fired the Portugese ammo in the old heavy OD plastic ammo packs for years. Not sure if I would use a lot of it, but the cases always seemed to be within tolerances so they would headspace properly.
 

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Safe?

Now, the GREAT thing is that if a rifle is chambered so that the bolt correctly closes on a 7.62mm GO gage at 1.6355" and does not close on either a 7.62mm NO GO gage of 1.6375" or the .308 Field Reject of 1.638" - then there are absolutely no worries about shooting either .308 nor 7.62mm in the rifle. This is how I chamber Garands and semi auto M14's for folks who want to shoot both .308 and 7.62mm surplus. (I chamber them so the bolt won't close around 1.635" to 1.636".)
I sent my SAI M1A in for repairs and they changed the headspace. I don't remember what it was before, but now it is 1.6325.

I want to shoot both .308 and 7.62.

Is .003 below the 7.62 Go Gauge a small enough difference to be insignificant or is it unsafe?

I hand-cycled some NATO surplus (German DAG) and the bolt fully closed.
 

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Interesting, I got automatically censored. When I typed a W, followed by a T, followed by an F, and posted it, the system automatically substituted "what the hey".

I guess the initials for a bad word are now considered a bad word... hey, that's a BAD LETTER. as in "I'm sorry Mrs. Johnson, we had to send little Johnny home from school today, because he used a bad letter".
lol, censorshit is getting way out of hand these days. I find it ironic especially on second amendment forums.
 

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...I am a bit confused, but I am easily confused...please bear with me...

...the tag that came with my new SAI M1A NM says it has been head spaced to GI specs and requires the use of GI mil spec or commercial equivalent...

...then I note your advice to not use NATO spec ammo in NM chamebers...it seemed that commercial 308 ammo would be ok though...???...

... the few rounds of ammo fired indeed show some mild symtoms of excess chamber pressure and are marketed as mil spec equivalent but are marked 308...

...how aboot the M118 load...???...is that correct for a 1.631 NM chamber...???...I am guessing that the M118LR load is not for use in this rifle which is a pisser as that is the type of ammo I had envisioned useing in this rifle...

...I guess I should have done this research before I ordered 440 round of remanufactored LC NATO spec 7.62x51 ammo...

...at this point I am feeling tightly headspaced...
 

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MGySgt USMC (ret)
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Discussion Starter · #36 ·
...I am a bit confused, but I am easily confused...please bear with me...

...the tag that came with my new SAI M1A NM says it has been head spaced to GI specs and requires the use of GI mil spec or commercial equivalent...

...then I note your advice to not use NATO spec ammo in NM chamebers...it seemed that commercial 308 ammo would be ok though...???...

... the few rounds of ammo fired indeed show some mild symtoms of excess chamber pressure and are marketed as mil spec equivalent but are marked 308...

...how aboot the M118 load...???...is that correct for a 1.631 NM chamber...???...I am guessing that the M118LR load is not for use in this rifle which is a pisser as that is the type of ammo I had envisioned useing in this rifle...

...I guess I should have done this research before I ordered 440 round of remanufactored LC NATO spec 7.62x51 ammo...

...at this point I am feeling tightly headspaced...
NM chambers are on the tight side of .308 spec, but they are still good for most GI or GI equiivalent ammo. Where problems can/will come in is if you use NON Nato Spec 7.62mm ammo.

As long as that ammo is remanufactured correctly, you should be OK.
 

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This is geting me suspicious.

Loading only for my two rifles, one a M1A and the other a M1 Garand in '06, I used GI head space gauges to determine case chamber relationship. It is my intention to avoid long case HS at all costs in either rifle..

The time spent and equipment at hand to use has left doubts, these doubts carry over to the actual firing of the rifles, it has developed into a lack of confidence issue to some degree.

After trying to follow all the excellent material in this Thread I have gone back to the basic method of trial sizing and hand closing the bolt... This seems a shame, but for some unknown reason it restored my confidence.

I would like to hear you comments on using that method on personal rifles..
 

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1. Do NOT use standard Nato Spec surplus ammo in a rifle chambered with a NM chamber and set up at the minimum headspace. Even if the rifle functions with no apparent problems, the chamber pressure of each round fired is significantly more than in a proper Nato Spec chamber.
...that was a fast reply...!!!...thanks for getting back to me...

...this is what caught me up on the first page...it seems in conflict to what you are saying here...

...I figgured I would have to forsake NATO spec loads and aquire a supply of the afore mentioned NM ammo...

...I had looked forward to aquiring some M118 ammo but remain unclear on whether or not it is suitable for my rifle...

...the load that showed flattened primers and slight cratering was PMC Bronze 147grn 308 that was also marked 7.62 NATO on the box...

...I have a small supply of Federal 308 WIN 165grn Bonded SP "Deer Thug" loads and some Hornaday 308 WIN 150 grn SST loads that looked like they would feed well through a gas gun and deliver superior termanal ballistics in various hunting situations...

...now I am having serious doubts aboot me ammo supply...live and learn, eh...???...
 

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loading only for my two rifles, one a m1a and the other a m1 garand in '06, i used gi head space gauges to determine case chamber relationship. It is my intention to avoid long case hs at all costs in either rifle..

The time spent and equipment at hand to use has left doubts, these doubts carry over to the actual firing of the rifles, it has developed into a lack of confidence issue to some degree.

After trying to follow all the excellent material in this thread i have gone back to the basic method of trial sizing and hand closing the bolt... This seems a shame, but for some unknown reason it restored my confidence.

I would like to hear you comments on using that method on personal rifles..
+1
 

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...well, the deer thug loads seem to be a bit hot bit not so bad as the PMC broze load in this rifle...

...I am wondering if laping the bolt will make this rifle a long range walter mitty rifle w/o messing up the accuacy too much...

...I am thinking the quality of the bore and the bedding and other stuff might have more bearing than how tight the chamber.is...
 
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