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Chamber specs USGI vs NM

2K views 4 replies 3 participants last post by  7.62bthp 
#1 ·
If I understand it right, manufacturers are bound to SAAMI specs....I have the following questions:(NOT just headspace)

1) What is the diff between SAAMI and USGI chambers
2) What is the diff between a SA Match chamber and USGI
3) There is a real diff in 5.56 and 223. Is the same true for 7.62 and .308.
4) what is the diff between 7.62 NATO and .308
I have gotten conflicting statements from Manufacturers, therefore the team here are considered the experts!!!
The reasoning here is the chamber on my New 2006 production SA NM is different than my 25yr old SA NM, which is different than my SA/USGI 30 yr old rifle. I can feel it big time on the resizing die, and was curious as wedge fit ammo can, on a cold day in hades, slam fire. Ive never had one, but homey don't want to go there!

Thanks
 
#2 ·
From the Fulton Armory FAQ page.

www.fulton-armory.com

What's the Difference between .308 Winchester & 7.62x51mm NATO?
by Clint McKee and Walt Kuleck
dumb question i alway thought these 2 ammos where interchangeable but some have told me otherwise whats the story??? jim

Hi, Jim,

This is a perennial topic, kinda like ".45 vs. 9mm" or "Best Guns & Loads for Deer."

They are not the same.

They are the same.

They are not the same, 'cause the .308 Win was released by Winchester several years before the Army standarized the T64E3 as the 7.62MM. You'll get an endless discussion of pressure specs, endless because SAAMI and the Ordnance Dep't measured pressure in different, unrelateable ways. Howver, the chamber drawings are different.

They are the same, 'cause nobody (and Clint's been looking for many years!) makes 7.62MM ammo that isn't to the .308 "headspace" dimension spec. So 7.62MM ammo fits nicely into .308 chambers, as a rule.

But in some 7.62MM rifles the chambers are long (to the 7.62MM military spec), notably the Navy Garands with 7.62MM barrels. Thus, using commercial ammo in such a rifle is not a good idea; you need stronger brass. Use military ammo or the best commercial only, e.g., Federal Gold Medal Match.

Most of the time it's a distinction without a difference. But if you intend to shoot .308 commercial in a military arm chambered for 7.62MM, first check the headspace with .308 commercial gauges first. You may get a surprise.

Best regards,

Walt Kuleck
Fulton Armory webmaster
Clint, What's the difference between .308 Winchester & 7.62x51mm NATO?

Jerry Kuhnhausen, in his classic Shop Manual (available from Fulton Armory; see the M1 Rifle Parts & Accessories or M14 Rifle Parts and Accessories Pages under Books) has published a somewhat controversial recommendation concerning .308 Winchester and 7.62x51mm NATO ammo, headspace & chambers. I broached the subject with him some months ago. He had his plate full, so we decided to chat on this in the future. When we do I'll report the results of our conversation.

I completely agree with Jerry that if you have a chamber with headspace much in excess of 1.636 (say, 1.638, SAAMI field reject), you must use only U.S. or NATO Mil Spec Ammo (always marked 7.62mm & with a cross enclosed by a circle) since the NATO mil spec calls for a far more "robust" brass case than often found in commercial (read .308 Winchester) cartridges. It is precisely why Lake City brass is so highly sought. Lake City brass is Nato spec and reloadable (most NATO is not reloadable, rather it is Berdan primed). Indeed, cheaper commercial ammo can fail at the 1.638 headspace (e.g., UMC) in an M14/M1 Garand. Many military gas guns (e.g., M14 Rifles & M60 Machine guns) run wildly long headspace by commercial (SAAMI) standards (U.S. Military field reject limit for the M60 & M14 is 1.6455, nearly 16 thousandths beyond commercial (SAAMI) GO, & nearly 8 thousandths beyond commercial (SAAMI) field reject limit!).

I also agree that 1.631-1.632 is a near perfect headspace for an M14/M1A or M1 Garand chambered in .308 Winchester. But I think that it also near perfect for 7.62mm NATO!

I have measured many, many types/manufacturers of commercial and NATO ammo via cartridge "headspace" gauges as well as "in rifle" checks. If anything, I have found various Nato ammo to be in much tighter headspace/chamber compliance than commercial ammo. Indeed, sometimes commercial ammo can not be chambered "by hand" in an M14/M1A with, say, 1.631 headspace (bolt will not close completely by gentle hand manipulation on a stripped bolt, although it will close & function when chambered by the force of the rifle's loading inertia), though I have never seen this with NATO spec ammo. I.e., if anything, NATO ammo seems to hold at the minimum SAAMI cartridge headspace of 1.629-1.630, better than some commercial ammo!

So, why set a very long 1.636 headspace in an M14/M1A or M1 Garand? It probably is the conflict mentioned above. Military headspace gauges say one thing, SAAMI headspace gauges say something else, as do the spec's/compliance covering ammo. In a court of law, who will prevail? I think Kuhnhausen gave all those who do this work a safe way out. However, I believe it not in your, or your rifle's, best interest. Whether you have a NATO chambered barrel (M14/M1 Garand G.I. ".308 Win."/7.62mm NATO barrels all have NATO chambers), or a .308 Winchester chamber, keep the headspace within SAAMI limits (1.630 GO, 1.634 NO GO, 1.638 FIELD REJECT). This subject is a bit confusing, and for me difficult to explain in a one way conversation!

Clint McKee
 
#3 ·
I'm at 1.632, but I'm more interested in overall differences, not just headspace. I've shot it all interchangebly for many years, so this is a topic of interest, more than concern. Bottom line, has SAAMI, and the manufacturers interest in avoiding lawsuits, "tightened" overall chamber specs. From my experience chamber leads, and base dimensions are usually different on military VS civillian rounds. Sounds like this may be a "controversial" subject base on the last post, so I'm bailing on this one!!!!
 
#4 ·
It's been said that one reason that a 7.62 NATO chamber is a little "longer" than .308 SAAMI specs is to allow for variations in ammo and also for the possible use of dirty ammo under battlefield conditions. I don't know!

Things get even more confusing when you add Match chamber specs into the mix. Match rifle builders have their own theories about chamber specs and many have custom ground reamers made to order and caution their clients to be very careful to use only ammo that's sized to their chamber dimensions. Neck diameter is a big variable in this case. There are custom chamberings like the M852 Match, the Palma '95, M118LR, etc, etc. Sometimes a chamber casting is the only way to be sure what you've got.

As for the average run-of-the-mill M1A/M14, this layman tends to agree with what Clint McKee has to say.

Al
 
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