Home Forums Hunting & Shooting Ask The Gunwriters CUP vs PSI

In my .284Win.

When I am running 50,000 or 55,000 CUP approxamately what am I running in PSI?

Thanks
Jim

Add ~ 10,000 psi to the cup units. The cup units are not really compatible

Or one forum says this, c.u.p. = (psi + 17,902)/1.51

Our own Denton sums it up nicely...

https://www.shootingsoftware.com/ftp/psicuparticle2.pdf

SAAMI's specs say 54,000 CUP max avg. pressure via copper crusher.
SAAMI's specs say 56,000 PSI max avg. pressure via piezo transducer.

See:

http://www.saami.org/specifications_and_information/publications/download/206.pdf

and

http://www.saami.org/specifications_and_information/specifications/Velocity_Pressure_CfR.pdf

Thanks
I know that PSI and CUP are two different measurements but they can be somewhat related compared.

Wouldn't it make more sense to compare velocity at top pressures? CUP and PSI may be different, but the velocity at max pressure will be the same.

That's probably the most accurate way for the home handloader to gauge it.

CUP and PSI are pretty much the same at some relatively low pressure, as I recall around 20,000, give or take a little. They start to diverge above there, and in most modern rifle cartridges with a SAAMI maximum average pressure of 60-65,000 PSI, 10,000 less CUP is about right.

But then pressure is a somewhat "moveable feast," to borrow a phrase from a guy named Hemingway.

Yes, he does.

There is a very simple equation contained therein..if you look for it. You can convert ANY CUP to PSI with his equation.

Jerry


BTW - I have the equation in 2 folders of my phone so I don't lose it.

SAAMI didn't change the spec on the 284 Win, they just measured the reference ammo with each of the two different systems (copper crushers and piezo transducers). In the example of the 284 Win, the numbers are nearly the same from the two different systems/methods. In other examples, they're much more different.

I struggle to see either wisdom or utility in using a conversion formula that is known to be vague and unreliable (or should I say imprecise?) when you can just look at the reference ammo measured each of the two ways, and see that in the example of the 284 Win, the max-pressure numbers are nearly the same regardless what measurement system you use.

The OP asked about the pressure range that is SAAMI-max, so I see no reason to cloud the issue.

Denton solves the problem for me.

I had a .284, also a 30-284. My son has two .284's, one for his wife and one for himself. We have worked out the velosity max. which is different for each.

What precipitated the question is we were looking at a reloading book that publishes the CUP specs for the .284. We did some projecting of Cooper Units of Pressure from the book max. to our max which, as you might guess, differs from the book. He asked me what I thought that might be in PSI. I guessed at a number and then thought there might be a better way - ASK THE EXPERTS! Thus my question.

I thought that the Algebra class I took in 1956 might be helpful some day.

Thanks men,
Jim

Good stuff there by Denton - memories of Drexel. Thx.
So, why is it that we don't all just go with PSI?

If you shoot 10 rounds, and measure and average the PSI, you'll get an estimate of what you'll get on future samples out of the same lot. When you shoot that second sample, the number you get will almost surely be different from the first. And the next sample will be different still.

So PSI does not perfectly predict PSI. And CUP does not perfectly predict CUP. Using my formula to predict PSI from CUP is no different. You get an estimate that is about as good as random measurement error and cartridge to cartridge variation allow. The formula works between about 30,000 PSI and 65,000 PSI. Below that, curvature takes its toll.

CUP, of course, is fading away. In time, it will become a historical footnote. Modern strain and piezoelectric methods are better.

As stated, the old CUP method was formerly taken as producing a true PSI number. If you read Ackley, he published what he thought were PSI numbers, taken with a copper crusher. That's why some his numbers seem so strange today. They are really CUP numbers. The CUP system does produce decent PSI numbers at lower pressures. At higher pressures it does not.

So, the formula is useful if you're trying to understand Ackley and other writers of that era, and it's useful if you're working up loads for a cartridge that has only a CUP specification. I ran into that when I instrumented my milsurp 6.5x55. I needed the equivalent PSI number, and couldn't find it.

A chronograph is your friend. In skilled hands, it is the most precise indicator of pressure that most folks have. Some loads in some cartridges will increase MV with increasing load in an orderly fashion. Others will begin to plateau. If your load plateaus, you should not continue to increase your load. I've run into this a couple of times. In one case, switching to a magnum primer fixed the problem.



That is the way that the world is heading. Before about the 1960s, equipment was not readily available to do PSI. We used CUP because that's pretty much all we had.

Don't confuse Ohairymikeus on the differences; he has trouble with the reductions made to the 44 Magnum, the 357 Magnum and how Speer has been slowly changing its data...