dig4gold

Something I've noticed in reloading my gas guns . . . there are different loading data for bolt action and "service rifle" guns. I have 2 books, Hornady and Speer and there is separate data for .223, and .308 gas gun vs. bolt.
I have also noticed that Hodgdon data runs a little hot! Trying to sell more powder I think.

steve4102

SAAMI does not have pressure specs for the 7.62, only the 308 and no the pressures are not different.

bronko22000

No Mercy - I really learn a lot reading your posts on the ARs. I appreciate reading them and applying what I've read. My AR-10 is shooting really good since I tailored a load to it. Some day I may get an AGB.

Ridge Runner

do your homework, the 308 is rated at 62k psi, the 7.62x51 is rated at 50k psi, there is a 24% difference, also a difference in the leade which can change things dramatically
RR

Nomercy448

You're makin' me blush, amigo

I do appreciate it, though, I love tinkering with AR's, and I'd be rude not to lend what little help which I can when I can. It also rankles me to no end to see so many factory models designed to be so over-gassed. Like I said - great for reliability, but he11 on brass! It works for most guys running factory loads of standard fare, but us reloaders get stuck tuning or rebuilding, or both.

super_hunt54

I guess I never thought about it because I've never owned an AR without an AGB so never ran across the problem.

steve4102

Lets do some homework together shall we.

First SAAMI has Max Average Pressure (MAP) for the 308 Winchester in both psi and CUP. They list the MAP for the 308 Winchester as 52K CUP and also 62K psi. Both are identical MAP, using different testing techniques.

http://saami.org/specifications_and_...wnload/206.pdf

In the 1950's pressure testing was done with the CUP method. They did not use piezoelectric strain gauge transducer method used today to calculate (psi). The US Military and others often times reported CUP pressure data as "psi" and have caused mass confusion ever since.

see here.

https://www.docdroid.net/shd8/the-tr...-nato.pdf.html

The real problem is the confusion between the old and the new methods of pressure testing. The old
pressure testing method used for the 7.62 NATO cartridge started out life in the 1950s and is still
published today in the US Army Technical Manuals. The figures are based on the copper crusher
method in CUP, but are published as PSI.
The new method is the piezoelectric strain gauge transducer method; it is the same technology used
today to show an automobile’s oil pressure. The piezoelectric strain gauge transducer pressure method
is a direct pressure reading based on an absolute standard, where the older copper crusher method a
conversion based on a relative measure and a conversion chart. And this is why you see the difference in
the pressure readings, but the older 52,000 CUP is equal to 62,000 PSI (piezoelectric transducer
method).
Today, these two methods are called CUP and PSI and the readings are different, but 52,000 CUP
equals 62,000 PSI and both are the same pressure, similar to the way 60 MPH equals 100 KPH.
To add even more confusion about the Ishapore 2A1, which started me on this article, many shooters
want to use the headspace specifications set by NATO, which is different from what the Indian Army set
for the Ishapore rifles.
In the figure below, you can actually see a page from an older reloading manual in which equivalent loads
are portrayed in both CUP (C) and piezoelectric transducer PSI (P).

Here is another article explaining the difference between psi and CUP in the 308/7.62.

http://how-i-did-it.org/762vs308/chamber.html

steve4102

Here is one such Military document where they listed the 50K CUP as psi. At the time this was published we were not testing in psi only CUP. so the psi designation in this document is incorrect, it should read CUP.

http://www.ar15.com/media/mediaFiles/348/32231.JPG

Note the 172gr bullet with 44gr IMR 4895 should produce 50K psi

When we open our manuals and look up this powder bullet combo we find this.



175gr bullet with 43.5gr IMR 4895 (.5 less) produces 59,400 psi, not 50K psi as reported my our Government.

Our government took CUP measuring numbers and listed them as psi, in reality the 7.62 and the 308 both run at a MAP of 52K CUP AND 62K psi.

steve4102

Now we can take all this confusion between CUP and psi and put into real life.

Here is a box of Federal 308 Winchester ammo.
https://www.midwayusa.com/product/95...oint-boat-tail


Technical Information

Caliber: 308 Winchester
Bullet Weight: 168 Grains
Bullet Style: Sierra MatchKing Hollow Point Boat Tail
Case Type: Nickel Plated


Muzzle Velocity: 2650 fps
Muzzle Energy: 2619 ft. lbs.

Here is a box of Federal 7.62 ammunition.

https://www.midwayusa.com/product/95...open-tip-match


Technical Information
Caliber: 7.62x51mm NATO
Bullet Weight: 168 Grains
Bullet Style: Open Tip Match
Case Type: Brass

Ballistics Information:
Muzzle Velocity: 2650 fps
Muzzle Energy: 2619 ft. lbs.

There is absolutely NO way that these two "different" cartridges can have the same velocity without running at Identical Pressures.

steve4102

Here is Winchester ammo.

https://www.midwayusa.com/product/29...l-metal-jacket

Technical Information
Caliber: 308 Winchester
Bullet Weight: 147 Grain
Bullet Style: Full Metal Jacket
Case Type: Brass

Ballistics Information:
Muzzle Velocity: 2800 fps
Muzzle Energy: 2560 ft. lbs.

https://www.midwayusa.com/product/29...l-metal-jacket

Technical Information
Caliber: 7.62x51mm NATO
Bullet Weight: 147 Grains
Bullet Style: Full Metal Jacket
Case Type: Brass

Ballistics Information:
Muzzle Velocity: 2800 fps
Muzzle Energy: 2559 ft lbs

Special note about this ammunition:
Winchester USA packaging marked 7.62mm

Again, same velocity = same pressure.

There is no way just changing the headstamp from 308 to 7.62 will change the ballistics. They are Indeed Identical.