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Old May 5, 2012, 07:41 PM   #11
SL1
Senior Member
 
Join Date: November 8, 2007
Posts: 1,999
Frumious,

"Working-down" from charge weights that produce pressure signs may be safe enough in strong guns, where the brass cartridge case or primer cup is the weak link. But, it is not a safe method for guns that can break at lower pressures than will RELIABLY produce recognizable over-pressure signs.

Also, just because an action is used for another, higher pressure cartridge does not indicate that you can load YOUR cartridge to the same PRESSURE.

The issue is the amount of force on the bolt face, which is a function of both the pressure and the SQUARE of the maximum internal diameter of the FIRED case.

As an example, let's consider the .444 Marlin, which is only about .430" inside diameter, while the .45-70 inside diameter is more like .480" (UNFIRED cases). So, assuming for purposes of illustration that those numbers are correct, the difference in force on the action for the same pressure is

(.480/.430)² = 1.246

So, if your gun is also chambered for the .444 Marlin, which has a maximum SAAMI pressure of 44,000 CUP, then the pressure in a .45-70 case that would give the same bolt thrust would be

44,000 CUP / 1.246 = 35,300 CUP

Now, that is a bit higher than the .45.70's SAAMI limit of 28,000 CUP, but it is nothing like 44,000 CUP. So, loading the .45-70 to .444 Marlin pressures might very well damage you gun.

Also, please realize that the numbers I used for illustration are probably NOT accurate enough to calculate the appropriate pressure limit for your gun. Due to the square of the diameter ratio, a little difference in either diameter can have a pretty substantial effect on the bolt thrust ratio.

So, I suggest that you stick with load data that has been approved for the gun you have, rather than try to "wing-it" with calculations like my illustration.

SL1
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