wally
Member
The Garand is .30-'06 by design, "remaking" it to .308 and having problems is on the makers, not the ammo!
Caliber question.308/ 7.62 NATO
- Thread starter igotta40
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Even if the handloader uses powder not fast enough to keep port pressures down to safe levels while peak pressure is still within specs?
Andrew Leigh
Member
You guys can debate this all you want to. In my world I reload 76 2 x 51 and 556 with dies Marked .308 and .223. I reload military cases and civilian cases and shoot them with different weight bullets in gas Rifles, bolt rifles, and blowback roller rifles without issue. That is my reality. This is the perfect formum topic. The differences may not matter but those of us that get hung up on those things can argue indefinitely.
Andrew, nice pictures, but there's a popular misconception shown.
All rimless bottleneck cartridge cases correctly dimensioned have headspace less than chamber headspace; there's a couple thousandths clearance. When fired, their shoulder is hard against the chamber shoulder. Firing pin springs drive the pin hard enough to set back shoulders a thousandth inch or more. There's a space between bolt face and case head called "head clearance" when such cases are fired.
As the case expands, it grips the chamber wall at the shoulder first (thinnest wall there) then moreso towards the back. That stretches the case body a little bit as the case body contacts the chamber body wall a little over time. It stops when the case head's hard against the bolt face.
Virtually all case rupures are about 1/10th inch forward of the case extractor rim as that's where the most amount of work hardening happens as cases are fired, resized then fired again.
With cases and chambers properly dimensioned, none are held against the bolt face by anything when fired. There's enough clearance to the extractor claw to let that happen. Otherwise, the extractor claw gets stressed by case rim contact and eventually will break from metal fatigue. There has to be room for case heads to slide into extractors when chambered. If there were forces somewhere to pull the case back, they would hinder case head easily going into battery on the bolt face.
All rimless bottleneck cartridge cases correctly dimensioned have headspace less than chamber headspace; there's a couple thousandths clearance. When fired, their shoulder is hard against the chamber shoulder. Firing pin springs drive the pin hard enough to set back shoulders a thousandth inch or more. There's a space between bolt face and case head called "head clearance" when such cases are fired.
As the case expands, it grips the chamber wall at the shoulder first (thinnest wall there) then moreso towards the back. That stretches the case body a little bit as the case body contacts the chamber body wall a little over time. It stops when the case head's hard against the bolt face.
Virtually all case rupures are about 1/10th inch forward of the case extractor rim as that's where the most amount of work hardening happens as cases are fired, resized then fired again.
With cases and chambers properly dimensioned, none are held against the bolt face by anything when fired. There's enough clearance to the extractor claw to let that happen. Otherwise, the extractor claw gets stressed by case rim contact and eventually will break from metal fatigue. There has to be room for case heads to slide into extractors when chambered. If there were forces somewhere to pull the case back, they would hinder case head easily going into battery on the bolt face.
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desidog
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It should also be mentioned that in pre-98 Mausers like the Spanish 1916 and FR-7, should not be shot with .308win because the 7,62 stamped on their barrels refers to 7,62 CETME (dimensionally the same as .308win) a 40,000PSI cartridge versus the .308 @ 60,000 PSI.
Sunray
Member
"...Yes, there's a difference..." Only since the Internet came along. Before there was such a thing, .308 and 7.62NATO were interchangeable. So were .223 and 5.56NATO. (The 'NATO' part is just a designation for a load. Isn't a different cartridge.) Literally millions of rounds of both chamberings were fired out of assorted milsurps and commercial hunting rifles with no fuss whatsoever.
SAAMI specs are about commercial hunting ammo only and do not apply to NATO anything.
"...port pressure..." In an M1 is another invention of the Internet. Came along when the assorted Garand pundits could not prove the claims of the ammo alone(primarily the bullet weight only) damaging rifles.
"... Many M1's were damaged by high port pressure..." What documentation proves that? Not anecdotal evidence.
"...Light Magnum, Superformance..." Same thing produced by Hornady. They say a 150 runs an MV of 3000 FPS. That's a tick over max loads.
SAAMI specs are about commercial hunting ammo only and do not apply to NATO anything.
"...port pressure..." In an M1 is another invention of the Internet. Came along when the assorted Garand pundits could not prove the claims of the ammo alone(primarily the bullet weight only) damaging rifles.
"... Many M1's were damaged by high port pressure..." What documentation proves that? Not anecdotal evidence.
"...Light Magnum, Superformance..." Same thing produced by Hornady. They say a 150 runs an MV of 3000 FPS. That's a tick over max loads.
someguy2800
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RecoilRob
Member
jonnyc
Member
Bart, feel free to comment on what I actually say, but that quote about .223s and 5.56s is not mine.
Andrew Leigh
Member
Bart,
From personal experience I will agree to disagree on this one.
I fixed a friends 7.62mm Israeli Mauser that was inaccurate. Brought the rifle from 6" to sub MOA. In this process I noticed that I was getting signs of case head separation after as little as 3 firings in Norma cases. Failures as early as 4 firings. After some research I found a technique to dial out the excessive headspace during sizing. This cured the problem and the owner now can utilise his case for 10 firing before he automatically discard these
Yes there is generally always headspace but when forming 7.62 brass with .308 dies the headspace problems are exacerbated. Here are some of the cases. I have never had case head separation on any other calibres, why? They all have headspace.
Many people shoot 7.62 and do not reload so the problem is not often observed
lysanderxiii
Member
I don't see how this is in disagreement with the post quoted....
The "chamber headspace"* minus the "cartridge headspace"** equals the "head clearance", period.
Excessive "head clearance" leads to head separation. If you resize the case with a longer cartridge headspace, the head clearance is decreased, and the brass lasts longer.
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* the distance from the bolt face, with the bolt in the locked position, to the shoulder datum diameter.
** the distance from the base of the cartridge to the shoulder datum diameter.
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