Is there a way to calculate pressure in cases?

[Larry Gibson]

If I was that concerned about pressure, I would use case head expansion measurements. I never load any of my older classic guns or modern replica anywhere near what is being talked about here. BP or the same pressure levels in smokeless only for me.

Any expansion of the case head will put the psi into a range way too high for the 1886 action. The case web expansion method could be used if a known load was available at a max safe pressure level for the 1886 action. Unfortunately I don't know of any in the pressure/velocity/bullet weight range the OP asked about.

[Kev18]

I was only asking because I thought it would be a good idea to have faster, bigger lead. The loads published are all recorded loads for my rifle.

[Norske]

The only tool most of us have to estimate pressure is our chronographs. When you reach either published powder charge or published velocity, whichever comes first, stop adding powder.

[303Guy]

I see the psi now (bottom of chart), the psi is higher than the cup and that is incorrect for the low pressure 44-40. I'll check the new program you send me and see what it says.

My pressure correlation indicates the PSI value would be higher than the CUP value but comes close around the 30,000 down to 20,000 mark. Remember that 4,830 PSI yields a CUP value of zero - that's the yield strength of copper but does vary from batch to batch. Actually, it seems that my correlation is yielding too low a PSI for the CUP value. I need to double check and update my data. It looks like 19,400 PSI correlates to 14,400 CUP? My correlation has them much closer together.

But the 44-40 data seems to indicate it the other way around. Could that indicate older data being measured differently?

[Savvy Jack]

My pressure correlation indicates the PSI value would be higher than the CUP value but comes close around the 30,000 down to 20,000 mark. Remember that 4,830 PSI yields a CUP value of zero - that's the yield strength of copper but does vary from batch to batch. Actually, it seems that my correlation is yielding too low a PSI for the CUP value. I need to double check and update my data. It looks like 19,400 PSI correlates to 14,400 CUP? My correlation has them much closer together.

But the 44-40 data seems to indicate it the other way around. Could that indicate older data being measured differently?

For the 44-40, 11,000psi is 13,000cup per SAAMI

My pressuretrace strain gauge tests yielded 18,000psi for loads that should be 20,000cup

There is no magical "linear" formula to convert cup to psi across the board. It can only be done in small ranges directly related to individual cartridge calibers. The 45-70 is 28,000psi and 28,000cup per SAAMI...so there is no rule of thumb formula for any other 45-70 loads. The .846 formula for the 44-40 is only good for a short range and is not reliable beyond about about 14,000psi....the spread could be more or less in either direction but.........

The only reason we have .846 formula for the 44-40 is because SAAMI gives us both cup and psi max load pressures for that cartridge. The secondary number we have is 22,000cup from Winchester's 1903 to 1945 High Velocity loads. Lyman lists load data for such loads for Group II rifles. Those loads produce between 18,500 and 19,000 psi in my tests of which Lyman claims up to 21,000cup for some of those loads. Sharpe's 1937 data also helps confirm such pressures.

[Larry Gibson]

At 20,000 to 24,000 psi and CUP they are close to the same but not always....the key word there is "close". Below that the psi is generally less and above that the psi is generally more. The key word there is "generally" as there are exceptions......

Truthfully I don't even attempt any correlation between CUP and PSI except as where SAAMI lists both as a MAP. Then that's only good at that representative amount for both.......

[Savvy Jack]

at 20,000 to 24,000 psi and cup they are close to the same but not always....the key word there is "close". Below that the psi is generally less and above that the psi is generally more. The key word there is "generally" as there are exceptions......

Truthfully i don't even attempt any correlation between cup and psi except as where saami lists both as a map. Then that's only good at that representative amount for both.......

exactly!

[303Guy]

Yes, I had a look at other data including Hodgdon's tables and what I figured is pretty much in line with what you have both said.

I found a correlation data graph and clearly the pistol CUP figures and rifle CUP figures do not align when plotted against PSI. CUP is a calculated figure using known deformation characteristics of the copper and apparently, each batch of copper crushers come with a calculation factor allowing for differences in the copper.

To me, the correlation is of interest just to bring the two together so I can get a better understanding of what given loads are doing. I am not going to create loads based on correlations, I'm going to use the actual load data and in a rifle rated for 45k CUP, it really makes no difference how the two measurement systems agree or differ below load chart values.

Playing with the Powley computer I see that the Powley PSI to CUP correlation gives a more pronounced difference than my correlation but checking load tables that give both indicates that mine is more accurate for my cartridge. There are fewer CUP values for the 308 but of those there does still seem to be a closer to mine.

I derived mine from a table of measured values that I found. My actual correlation calculation gives a R² of 0.9908 only means that my calculation fits the given values well, not that the values are true.



What this does show is that CUP values are not very accurate which is probably the only value of this chart.

Anyway, it was a fun exercise.

[Larry Gibson]

303guy

"What this does show is that CUP values are not very accurate which is probably the only value of this chart."

Have to agree 100% with that! CUP is only a measurement of the maximum psi that crushed a copper pellet so much. Not very precise at all. It also is a very slow and expensive system to use and there was no standard as to the size of the test sample back when most CUP measurements were actually made. To further exacerbate the issue is when older CUP measurements are listed as "psi".......

[Savvy Jack]

303guy
To further exacerbate the issue is when older CUP measurements are listed as "psi".......

Yeah, this is always an issue until one understands when the piezo was started. Back then that is all they had spo psi was psi cause they didn't have to differentiate between that and anything else. Just like gun powder. Once upon a time the only "gun powder" was "black powder". Gives me a headache!

[Norske]

I used to use a software called "Load from a Disk", which was the Homer Powley "computer" for a computer. I had faith in it for IMR powders but not so much for double-base powders. Homer's slide rule was based on single-base IMR powders.

[Larry Gibson]

The Powley slide rule used "psi" derived from CUP so it's not applicable to "psi" as used today derived from piezo-transducer/strain gauge measurement. Also, the single based IMR powders generally increase pressure in a linear fashion as the charge is increased. Double based and ball powders most often do not, especially at higher end pressures.