CUPs compaired to PSIs

[J E Custom]

I still cant fine any direct comparison between the two and It would be nice to have a chart/formula
that would compare them.

I have acquired many loading manuals over the years and some have tested CUPs in pressure test barrels/gun, Most if not all load data now days is all calculated.

Some have tried and true real world loads in cartridges and barrel lengths. I still use some of them
and I would like to be able to know what PSI they were.

Reading the brass is not a very good way to work up loads and buy the time you see signs of pressure you have gone to far in most cases and with the different brands of brass comes different harnesses making it very un reliable.

Just thought I would find out if anyone could come up with a good comparison that was tested against CUPs that could give accurate comparisons to PSIs.

This is the best info I could find.
CUP vs. PSI — What's The Difference in Pressure Measurements « Daily Bulletin

Any help would be appreciated.

thanks

J E CUSTOM

 

[MagnumManiac]

I still cant fine any direct comparison between the two and It would be nice to have a chart/formula
that would compare them.

I have acquired many loading manuals over the years and some have tested CUPs in pressure test barrels/gun, Most if not all load data now days is all calculated.

Some have tried and true real world loads in cartridges and barrel lengths. I still use some of them
and I would like to be able to know what PSI they were.

Reading the brass is not a very good way to work up loads and buy the time you see signs of pressure you have gone to far in most cases and with the different brands of brass comes different harnesses making it very un reliable.

Just thought I would find out if anyone could come up with a good comparison that was tested against CUPs that could give accurate comparisons to PSIs.

This is the best info I could find.
CUP vs. PSI — What's The Difference in Pressure Measurements « Daily Bulletin

Any help would be appreciated.

thanks

J E CUSTOM

J E, I know this can be perplexing, not knowing the conversion between CUP & PSI, but, I would also like to throw in another curve ball.
Depending on WHERE you place the piezo crystal also changes the PSI given.
I have a pressure trace, I use it to work up loads in absolute numbers, no fudge factors, no calibration, just raw data. Then I load 10 and take an average.
I read that CIP take their measurements from the case mouth, while SAAMI take there's from a mid point along the case. I have tried both points, and yes, the measurements are vastly different.
I have not tried NATO testing because I cannot find a definitive description of what exactly they do.
I know they test back thrust with an oiled case, but, chamber pressures are not clearly defined as to their method.
The readings at the case mouth are higher, as indicated by CIP readings.

Getting back to the original question, I recall reading that 50, 000 CUP was roughly 60, 000 PSI.
I also remember reading that CUP was so unreliable that no 2 tests ever gave the same readings, even in the same test barrel.
Why do you want to know the pressure?
It never is the same as the test barrel anyway, have proven this many times now, same charge weight, same bullet, same case, same primer, different velocity to book and different pressure. Also, most brands of brass don't show ejector marks until 70, 000psi is reached or exceeded. Some primer lots have flattened at 55, 000psi, a full 10, 000psi below max pressure.

Cheers.
gun)

 

[Mikecr]

I agree with MagnumManiac.
Ultimately, MY barrels are unique, and so are yours(each one of them).

I rely on QL's reporting of pressure in PSI, but only where my load results are matching QL results(calibrated). Reported pressure is not a problem for me though -until it really is, which I will measure at the weblines.

 

[J E Custom]

The reason I wanted to know is that some old loads listed were( Example ) 52.000 CUPs the same load now may run 65.000 PSI in the new manuals and that seems to be an extreme difference.

With changing brass and powder I just wanted a feel for where I was at with the old loads.

It used to be that 55,000 CUPs was maximum and sometimes over maximum and with the new cartridges that were rated at 65,000 PSI That had no better performance I though it was in reality, about the same pressure even though the numbers were way apart.

I realize that the CUPs were not absolutely perfect, but they were all we had at the time and was an indication of the pressure based on barrel length, powder type and charge, bullet weight, primer type and seating depth.

I also received a reply from one of the members that really helps and I will share it with those that are interested.

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

It also needs to be noted that a lot of the older rifles are/were rated using CUPs and they are much lower, 43,000 to 45,000 CUPs and it is important to not over pressure older actions so I was looking for some comparisons. A good example is the 45/70 trap door, all loads for it are below 40,000 psi
but in modern rifles it can be loaded from 55,000 to 65,000 psi.

Anyway, it is worth learning about and understanding. so thanks for the feed back.

J E CUSTOM

 

[Ridgerunner665]

40,000 psi will completely disassemble any 45-70 Trapdoor that I've ever known..... There are 3 levels of 45-70 loads.....Trapdoor (28,000 psi), modern lever action (40,000 psi), and Ruger #1 loads (50,000 psi).

Those Ruger loads put the old warhorse nipping on the heels of the 458 Win Mag, and will knock the $&$# out of the wherever is on both ends of the gun.

And this "correlation" you're looking for doesn't exist... You might find something to get you close within the realm of a single round, but across rounds of different sizes and shapes..... It doesn't exist, it can't.

 

[J E Custom]

40,000 psi will completely disassemble any 45-70 Trapdoor that I've ever known..... There are 3 levels of 45-70 loads.....Trapdoor (28,000 psi), modern lever action (40,000 psi), and Ruger #1 loads (50,000 psi).

Those Ruger loads put the old warhorse nipping on the heels of the 458 Win Mag, and will knock the $&$# out of the wherever is on both ends of the gun.

And this "correlation" you're looking for doesn't exist... You might find something to get you close within the realm of a single round, but across rounds of different sizes and shapes..... It doesn't exist, it can't.

You are correct about the Trap door, It was a typo and should have said 40,000 CPUs not PSIs

There are also some other rifles used for the 45/70 that will handle modern cartridges Like the 1885/B78 Browning I loaded the 1885 to almost the same velocity as a 458 win and never had a worry. There is also a Siamese Mauser (Bolt action) that can take the pressure.

J E CUSTOM

 

[Sully2]

Everything Ive read in more that 45 years stated there IS NO direct correlation in CU and PSI. Its like apples and oranges.

 

[6.5Swede]

Everything Ive read in more that 45 years stated there IS NO direct correlation in CU and PSI. Its like apples and oranges.

I've been told the same thing from reliable sources, there is no direct correlation.

Steve

 

[J E Custom]

Everything Ive read in more that 45 years stated there IS NO direct correlation in CU and PSI. Its like apples and oranges.

You are correct.

I knew that different cartridges had a different/inconsistent relationship between CUPs and PSIs
but I wanted to have something to see what differences there were to be able to use some of my older loads with confidence.

Most new loading manuals are very conservative and people tend to start loading at or near max because of it. In comparison, the older manuals have loads that are truly max and should be approached with caution. with the changes in cartridge designs that people are making starting loads should be lower if anything but SAMME specifications are used and even then it is still recommended
to start low.

I have found a good guideline that works, and SAMME has set a Standard comparison that will help.
I now have a good grasp of the differences and feel more comfortable with them.

Some of the revised SAMME pressures are now lower and I agree that They should be from past loading experiences.

Anyway, Thanks to all for posting.

J E CUSTOM

 

[MagnumManiac]

You are correct.

I knew that different cartridges had a different/inconsistent relationship between CUPs and PSIs
but I wanted to have something to see what differences there were to be able to use some of my older loads with confidence.

Most new loading manuals are very conservative and people tend to start loading at or near max because of it. In comparison, the older manuals have loads that are truly max and should be approached with caution. with the changes in cartridge designs that people are making starting loads should be lower if anything but SAMME specifications are used and even then it is still recommended
to start low.

I have found a good guideline that works, and SAMME has set a Standard comparison that will help.
I now have a good grasp of the differences and feel more comfortable with them.

Some of the revised SAMME pressure are now lower and I agree that They should be from past loading experiences.

Anyway, Thanks to all for posting.

J E CUSTOM

One word of caution, SAAMI changed their testing parameters on pressure. In 2010/11, thet changed from using an average of pressure data for maximum charges to absolute pressure data, this is why maximum charges have changed. Every test I do with my pressure trace on average pressure ALWAYS results in a higher max charge. If I test single cases at each charge level, the max is lower than the average.
This is a significant difference. It makes a huge difference to max listed loads in modern manuals.
You may think they're conservative, but, what's max is actually max in those manuals.

Cheers.
gun)

 

[J E Custom]

One word of caution, SAAMI changed their testing parameters on pressure. In 2010/11, thet changed from using an average of pressure data for maximum charges to absolute pressure data, this is why maximum charges have changed. Every test I do with my pressure trace on average pressure ALWAYS results in a higher max charge. If I test single cases at each charge level, the max is lower than the average.
This is a significant difference. It makes a huge difference to max listed loads in modern manuals.
You may think they're conservative, but, what's max is actually max in those manuals.

Cheers.
gun)

More good information !!! Thanks.

I almost bought a lab model chronograph that had strain gauges but decided not to(Wish I had Now)
and this would not be a question.

I am very conservative now and don't push any cartridge. Reading pressure using the brass/primer is not an exact science so
I don't rely on that method.

Loading manuals are more of a guideline to get you started than anything else Because they wont tell you which primer, powder or how much powder, you rifle will like. That requires testing.

J E CUSTOM