Specs: 7.62x51mm versus .308 Win w/ diagrams (MERGED)

[Uncle Buck]

In the February 2008 issue of American Rifleman, page 20 , the editors deal with the ".308 vs 7.62, what's the difference" question. They say:

"Unlike using 5.56 x 45 mm NATO ammunnition in .223-chambered guns, which the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) lists in its "Unsafe Arms and Ammunition Combinations" there is no such blanket prohibition on using .308 factory ammunition in 7.62 x 51 mm rifles or visa versa"

They also state:

"Also, military 7.62 x 51 loads can be encountered that exceed SAAMI's .308 Win maximum pressure of 62,000 psi."

So here is another claim in print that 7.62 ammo may be loaded to pressures higher than .308. This is just the opposite from what you hear in the net rumor.

[blameitonfinland]

is 308 safe in an L1A1? I've used SB without issue

[ammolab]

Originally posted by blameitonfinland
is 308 safe in an L1A1? I've used SB without issue

As stated above by SAAMI and American Riflemen technical writers....the rounds are interchangeable. The use of .308 should not present you with any new safety issues at all.

[blameitonfinland]

thanks

[EricCartmanR1]

I love this thread!

[jkeen579]

I have been discussing this with a guy I work who is dead set against shooting factory 308 in 7.62 guns. He is claiming that it is the pressure put on the gas system parts due to the different powder. He is saying that the burn rate on factory is faster than military, thus, causing excessive pressure in the gas sysyem. Is there anything I can tell him that will rufute that?

[ammolab]

Originally posted by jkeen579
I have been discussing this with a guy I work who is dead set against shooting factory 308 in 7.62 guns. He is claiming that it is the pressure put on the gas system parts due to the different powder. He is saying that the burn rate on factory is faster than military, thus, causing excessive pressure in the gas sysyem. Is there anything I can tell him that will rufute that?

The powder in .308 Win ammo may well be "faster" burning than that in 7.62NATO ammo......it may well be SLOWER. Various burn rate powders are used to load 7.62NATO as well. You and I (and your friend) do not know which powder is selected to load any given lot of ammuniton.

His statement is somewhat a mistruth....as a SLOWER powder is the one which will give you a higer gas port pressure and strain the function of a semi/full auto gas powered weapon. That said...only the "Light magnum" .308 Winchester factory rounds would have a powder slow enough to cause much concern. I think these rounds have a cautionary warning on the box....maybe that is where your buddy got his 'education' on this subject.

[brownknees]

Powder burn rate tables are not to be trusted as a reference source, see any reloading manual for confirmation of that.
Also powder is a propellant, not an explosive why does this matter? Because powder burn rates vary with pressure, that's why they are called "Progressive" powders.

If he doubts that trickle a 4" long line of powder on a hard, non-flammable surface & light it. Fizz, fizz, but no BANG! then just say "Is that a fast, or slow powder?"

[Uncle Buck]

NATO has recently updated their standards for 7.62 x 51 according to NATO AC/225 (LG/3-SG/1) to EPVAT testing.

The new standard now uses a Kistler 6215 Piezoelectric Transducer gauge and has a maximum pressure of 60,190 PSI (as opposed to 50,000 PSI CUP).

http://en.wikipedia.org/wiki/NATO_EPVAT_testing

This is much more comparable to the SAAMI and CIP pressure specs. The ammunition hasn't changed, just the type of gauge and the pressure specs. Now you can compare apples to apples.

UB

[JamesTheScot]

So why any difference at all between x51 and .308 using the piezo transducer?

The old x51 CUP figure was less than the SAAMI .308 CUP figure ostensibly due to the different locations and methods of mounting the copper crusher to the chamber.

So is there a difference then in how NATO attaches the piezo transducer versus how SAAMI does it? I know that 1,810 PSI isn't much, but the fact that there is a difference still makes one wonder a little bit if NATO rifles are not as strong as .308 rifles. Sometimes, the devil is in the cumulative wear on parts, not just in a sudden catastrophic failure.

I'm a convert and think it's OK to shoot .308 in a good x51 chamber, but it's still a little disconcerting that no one in the ammo industry seems to be willing to take a stand. I suspect that there's some uncertainty there as well and they aren't comfortable endorsing the use of their ammo in milsurp rifles.

I'm starting to look at this more like the +P issue with .38's. Some modest use of .308 is OK but a steady diet might loosen things up a bit?

Thoughts?

[brownknees]

After a couple of thousand rounds of both I'm finding no appreciable wear.

[Uncle Buck]

Originally posted by JamesTheScot
So why any difference at all between x51 and .308 using the piezo transducer?

It could be a number of things:

1. A real difference, but still not a very significant one.

2. A definition difference. Some use maximum pressure for individual cartridges, while others use a "maximum average pressure", where the maximum is averaged over a number of individual measurements, some of which are above and below the average.

3. Gauge difference. NATO uses the same gauge and measurement methods as the European CIP standard and the pressure maximums are exactly the same for NATO and .308 CIP.

The apparent difference between NATO and SAAMI (approx 2%) is not of the same magnitude as the difference between +P and standard pistol ammo which is more like 10%.

[lazersnthjungle]

However complex, I'd have to side with the fact that the pressures are "essentially" the same. The greater issue still may be the headspace. Do you suppose a visual inspection of a commercial .308 Win case from a 7.62 chamber would reveal any distortion or other signs of distress?

[brownknees]

Not with any of the 1,000s I've fired.
Visually you won't see a darned thing, the difference is way to small to be visually aparrent. I've used the Stoney point gage with a caliper & can't find diddly either.
In reality the chambering & extraction process is violent enough to hide the thoeretical differences between the two as the stretching & re-headspacing on chambering is bigger than the difference!

[lazersnthjungle]

Soo . . . I'm thinking the Comm. .308 cases just won't stand up to the reloading as well . . .(?)

I've been casting about on the +P .38 analogy since I often carry a snubby and have (probably) overshot it too many times with +P.

Best I can figure, the pressure differences we're seeing with .308 and 7.62 may be 3% (at most). But +P .38 is at least 10-12% over MAXIMUM pressures. It is more like 60% over standard pressures.

The +P .38 thing is admittedly off-topic, but since the analogy was cited, I think it fair to level the playing field.

The difference between .308 Win and 7.62x51 pressures are minuscule compared to standard .38 special and +P.

[brownknees]

Soo . . . I'm thinking the Comm. .308 cases just won't stand up to the reloading as well . . .(?)

In something like a FAL the cycling process beats up the brass so much that's the limiting factor for either case. When I reloaded both for a bolt action I could get 10~12 loads out of either. It seems to be a question of itty-bitty differences in headspace, once you've resized once to the "correct" headspace the wear is limited so there isn't much difference really.

Which bring on the thought that if this made a real difference you'd be able to buy either 7.62, or .308 Win. dies, & for some reason you can't maybe another indication of the difference being thoeretical only.

[lazersnthjungle]

Good point on the dies!

I don't guess my M1A would beat up brass any more or less than an FAL (?)

[brownknees]

More like differently
My M1a put a diagonal mark on rounds 1 thru 19, it didn't look as bad as the creased necks of the FAL, but it did shorten case life. ON the bright side round 20 was cherry