Thursday, August 07, 2014

My contribution to the 'Is there any difference between (updated)

7.62x51 and .308?' is this from the folks at Bulk Ammo
The only specifics I can ever remember is that military brass tends to be thicker-walled than commercial, thus the same propellant charge tends to give higher pressures.


Update: received this from this gentleman:
A few years back a government document was revised from "CUP psi" to just read "psi" and this wrong information has been repeated over and over again by folks who should know better.

When you measure 7.62x51 or 308 Win by the same method, you get the same pressure standards.

The REAL difference is in "port pressure" where milspec ammo has a requirement to meet, but commercial 308 Win does not. This is really only an issue with oprods for M1As and converted Garands
.

2 comments:

AM said...

Pressure difference FAIL.

A few years back a government document was revised from "CUP psi" to just read "psi" and this wrong information has been repeated over and over again by folks who should know better.

When you measure 7.62x51 or 308 Win by the same method, you get the same pressure standards.

The REAL difference is in "port pressure" where milspec ammo has a requirement to meet, but commercial 308 Win does not. This is really only an issue with oprods for M1As and converted Garands.

But try to convince someone that what they know is wrong because you can't reliably convert between two pressure systems....

Keith said...

To second what AM wrote;

The "pressure" difference can be accounted for by the different techniques. A scan of the SAAMI pressure testing manual / standard is probably up on "Steve's pages" .

The variation comes down to
1) the inertia of the piston and deformation of a metal crusher - compared to the almost instant response of an electronic sensor (which might still be non-linear)

2) the location of the port for bleeding off the gas - chamber or throat.

That said, there are some otherwise very similar rounds with drastically different pressures.

Common examples would be the .280 Remington and .222 remington (and the long gone but excellent .222 mag), which operate at around 40,000 PSI and have reputations for being easy to get good accuracy from

and the .270 Win and .223 rem, which operate at magnum pressures and can be difficult to get to shoot to the same standard.