For years debate has gone on about exactly what is the “safe” pressure levels for small ring Mauser actions.
Someone I respect, Larry Gibson, has used his ballistics lab to produce some data. Larry and I have been arguing about the relative safety of small ring Mausers for fifteen years now, and I’ve come to the conclusion that “He’s may be right, but I’m not wrong.”
You can see Larry’s original post here: https://goodsteelforum.com/forums/topic/308w7-62-nato-psi-vs-7×57-psi-for-use-in-sr-mausers/
For the 7×57:
The SAAMI MAP is 51,0
The C.I.P. PTc maxis 57,0
Both are with transducer measurement.
Commercial Factory and hand loads:
Federal 175 RNSP; 49,9 2400
Rem-UMC 175 RNFMJ; 57,2 2311
Remington 175 RNSP; 48,2 2393
Winchester 175RNSP; 50,2 2376
My standard load 175 RNSP; 49,2 2334
Hornady Light Magnum 139 SPBT; 44,5 2624
My standard “light” hunting load 154 SP; 55,7 2579
CAVIM (Venezuela) 139 FMJBT; 46,0 2590
PS 1950 (Spanish) 154 FMJBT; 59,8 2442
PS 1951 (Spanish) 154 FMJBT; 60,5 2543
FAMME (Chile) 133 FMJBT; 55,3 2718
DWM 1918 (German) 172 Cupro RNFMJ; 54,8 2295
This data, combined with pressure data for surplus and commercial 308 Win and 7.62×51, convinced Larry that the Small Ring Mauser’s are essentially safe to shoot with 7.62×51 ammunition, because so many of the 7×57 loads tested were above the 51k PSI SAAMI max.
This is where I disagree with Larry. All of the new manufactured commercial ammunition in his data set are below 51k PSI except for the Rem-UMC 175 RNFMJ load, which is pretty dang old. The Rem-UMC headstamp was discontinued around 1959 and transitioned to the R-P headstamp for “Remington Peters” so whatever ammunition Larry shot, it was at least 57 years old based on possible latest date that Remington could have made that particular headstamp on 7×57.
So, with that data in mind, lets ask ourselves why all the older ammo had a much higher max average pressure then new commercial ammo, despite having essentially the same external ballistics? The answer lies in the chemistry of smokeless powder.
Single base smokeless powder is a mixture of nitrocellulose, binders, preservatives, and burn rate conditioners. The purpose of the preservatives is to be “sacrificed” as the nitrocellulose breaks down over time, and one of the byproducts of this breakdown is nitric acid. Nitric acid is will etch the inside of a brass case and even eat all the way through it over time. The other thing that it will do, is once the preservatives are eaten up, is go after the burn rate conditioners which changes the burn rate of the powder.
So, how does powder that has no preservatives and fewer burn rate conditioners perform? It performs like a faster burning powder and for the same charge weight as a slower burning powder you get a faster and higher pressure spike. This is why lots of ammunition are regularly tested, to see if the powder is still within operating specifications, and some lots last a long time (I’ve shot off 7.62×51 from the late 70s not even a decade ago as it was issued to me, and some 50 BMG literally from the Korean War era). Other lots…go bad and get demilled and sold off as components much quicker.
But, and this part is important, the amount of energy contained in the gun powder doesn’t increase. This means that the maximum amount of energy that this old powder can impart to the projectile remains roughly about the same (some power is lost as the powder degrades). So you get a higher initial pressure spike, but then also a more rapid pressure decrease as the bullet goes down the bore and out the muzzle. This is why Larry’s data can show an almost 9k PSI difference in pressure (the max pressure) and less than 80 fps in velocity difference (with the lower pressure load being faster). If you run into a situation where a powder didn’t get 100% burn before leaving the bore, you may see an increase in velocity as the powder burns faster and you get 100% burn before the muzzle is unsealed.
Now once the powder has really degraded, you begin to have significantly less and less energy in the cartridge, and so you start seeing more and more significant velocity losses. You also begin to see many more hangfires and duds as the remaining powder gets harder and harder to light off as so much of it has degraded to the point of being mostly inert. Even Larry’s M80 ball data shows this:
M80 Ball WRA 68; 5 2,8 2790 (oldest is slowest, most powder degradation, lowest MAP and velocity, might still be using IMR stick powder)
M80 Ball LC 87; 61,2 2924 (next oldest, over NATO EPVAT pressure, nitric acid eating those burn rate inhibitors, velocity higher than specified for the load)
M80 Ball LC 90; 57,3 2852 (only 27 years old, not too bad)
I wish we had more data to discuss, but the data Larry presented is consistent with the published literature about pressures rising with age until they start falling.
The other reason that I disagree with Larry about what is an acceptable pressure level for a small ring Mauser is the gas handling capabilities of the actions themselves. You can add a flange to redirect gasses, cut a gas port in the receiver ring and bolt, and several countries did at least the gas port cutting on some of their small ring Mausers.
And here’s why they added those upgraded bolt shrouds and cut gas ports, brand new Federal Gold Medal Match can still suffer a catastrophic case head failure…
If you get a full or partial case head failure on a small ring Mauser, there is dang near nothing stopping all that hot gas from pushing directly down the left receiver rail along the bolt and shooting right back into your face. If you shoot a small ring Mauser, and you don’t wear quality Z87 or better safety glasses while doing so, I highly recommend that you start immediately.
And that’s the real reason why the old “45k CUP” or “51k PSI” limit comes from, brass is less likely to fail at lower pressures than higher pressures.
So, unlike Larry I won’t tell you to go out and rechamber your Spanish M93 into a 243 Win or 260 Rem and run max SAAMI pressure loads through it. Larry is right that many people have done it, the 308 Win conversion was very popular at one point. But I’m not wrong either, those rifles were designed in the late 1800s and the reason why the small ring Mauser platform was abandoned for the M98 Large Ring Mauser platform was because they had obvious design deficiencies that showed up in actual use.
Here is a picture of an M96 swede bolt with both bolt lugs sheared off:
Losing both front lugs is the reason why the M98 action has the third safety lug at the rear, to stop bolts from flying back and killing the shooter.
Yes, you can blow up any rifle. But different rifles handle failures more or less gracefully than others. The small ring Mausers can give you a lifetime of good service, and you may, like Larry, have no issues whatsoever going through multiple barrels on a single action. Or it could go the other way, which is why I recommend keeping the pressures down. The 6.5×55 and 7×57 have killed game cleanly (and cleaned targets) for over a century without being high pressure hot rods, and there is no need to start today.
And if you want a long range target or hunting rifle, there are much better and cheaper actions to build on than a small ring Mauser. But, it’s your money and your life, and all I can do is state my case for loading conservatively.