In looking at the Lyman cast bullet handbook some of their pressures are given in PSI and others are in CUP.    I know that the PSI is almost always higher than the CUP.    I saw a formula on the www but in trying it with real numbers it made no sense at all.  Is there any accurate way to convert CUP to PSI??? 

George

The problem is, it's non-linear because copper work hardens as it deforms, and because dynamic deformation also depends on the pressure curve, i.e. the ramp rate and length of time the pressure is applied, not just the magnitude.  CUP was intended to be a close analog of PSI, but an easier way to measure and retain the measurement, in a deformed copper slug that could be measured at any time after the test.  It is probably very close to exact at one pressure range and with some powders, where the pressure calibration was derived at.

The reason published values also don't correlate between psi and cup is that the pressures measured don't just depend on the bullet weight, primer, powder and charge weight, they also very much depend on the barrel characteristics, the brass used, and the chamber.  To correlate, they'd have to be tested in the same chamber and barrel, and obviously different pressure methods don't use the same equipment.

So, pick a number like craigster has, based on powders and cartridges similar to what you are using, and go with it.

Also, recognize that pressures in your particular rifle may not resemble those in the test equipment either, because of different characteristics of your chamber and barrel.

The ratio isn't linear, there is a curve, with a cross over point in the ~35,000 - 28,000 CUP/PSI range.

I have or have seen a chart but, couldn't find it so, I went to the SAMII site and took their pressure std for the following cartridges:

6mm Rem 52k CUP % PSI 65k = .80
7x57 46k % 51k =  .90
8x57 37k % 35k = 1.06 
8mm Rem Mag 54k % 65k = .83 
300Sav 46k % 47k = .98
30'06 50K % 60k =  .83
30/30 Win 38k % 42k = .90
35 Rem 35k % 33.5k = 1.04 
44/40 13k % 11k =  1.18
45/70 28k % 28k = 1.00 

There are inconsistency's in the above but, that is how SAMII rates each cartridge max CUP to PSI. We work in the rang of the bottom two.