Thumbing through a '70's vintage 'NRA Handloader's Guide' last nite, I noticed that the maximum average pressure for both the 357 Mag and 44 Mag were listed as 46,000 CUP. While checking out Ramshot Powder's latest Load Guide, I noticed the SAAMI 'Recommended' Max PSI Level as 35,000 PSI for the 357 and 36,000 PSI for the 44, both of which seem a little anemic. Some of the loads listed in the NRA guide for THOSE TWO cartridges in particular are a LOT higher than what you see today.

I've seen a lot written about conversions from CUP to PSI, but, as far as I know, there is no absolute method of converting one to the other. I'm curious as to what 35,000 PSI in the 357 would equate to in CUP, but, I seriously doubt it would be 46,000, and I've never seen an instance where, when both are listed, the CUP number is higher than the PSI number.

As a point of comparison, I have an Alliant manual from a few years ago, which lists a max load of 10.7 Blue Dot as producing 1,445 FPS with the 170 Sierra at 41,700 CUP; the latest manual shows 9.7 Blue Dot to be the max, producing 1,310 FPS with the 170 Sierra and 33,800 PSI.

It seems that we are REgressing instead of PROgressing.

R-WEST

It's all about sales, that's so you will "need" a 454 Casull or a .480 Ruger

MM

I noticed the same thing a few years ago when I got a new manual. The old loads for the magnum revolvers were running around 54,000 psi (or 43,500 cup). There were occasional remarks about a "steady diet of full power loads may loosen things up", and that was one problem. The other is that 50,000 psi is close to the limit on a revolver and its hard to estimate that max from traditional pressure signs, so the manuals have been backing off.

There is a very simple method for converting back and forth between CUP and PSI.

PSI = -17902 + 1.51 x CUP

Many people who should know better have written that you cannot convert from one to the other. That is rubbish. The alpha risk on this calculation is .000, and the R^2 is .927. That removes all doubt about whether they are correlated, and whether you can connect them with a formula. They are. You can. The math has been done, and leaves no doubt.

All that said, you do have to recognize the limits to accuracy. Both the PSI piezoelectric and crusher methods of measuring peak pressure have a considerable amount of random error. The random error in the crusher method has a standard deviation of around 1,300 CUP or 2,000 PSI. Average ten shots, and that random error will come down by a little more than a factor of 3. It's still not very precise.

The standard deviation of the total error in the formula I gave above is about 3,000 PSI. That means that 68% of the time the error in the formula will be less than 3,000 PSI, and 95% of the time it will be less than 6,000 PSI. The error is significant, and from many sources, including the random error in both pressure measuring methods mentioned. The good news is that most gun designs will handle some margin of error. The bad news is that the published pressure data is NOT terribly precise. The measurement systems used just "don't have it in them." That is part of the reason that maximum loads vary so much from manual to manual.

If anybody is still interested in this topic, there's some interesting debate going on over at http://www.huntchat.com/showthread.php?s=&threadid=16784&pagenumber=1

R-WEST

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"the spotlight of truth will cause the cockroaches of deceit to run for cover every time"
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