Correlating psi and c.u.p in the 45 Colt?

[singleaction]

SAAMI max pressure for standard 45 colt loads is 14,000psi or 15,900 CUP. I understand that psi and CUP numbers tend to be closer and more consistent across broad pressure ranges, compared to bottlenecked cartridges. There is a now lot of +P data out there in psi and cup. With the introduction of the mid-frame Blackhawk, with its 23k psi top end load level, I would like to know what 23k psi translates to in CUP. I've asked a similar question before, but no satisfactory answer was given. I have a long mid-frame Bowen cylinder that should be equal in strength to the shorter factory mid frame cylinders, but will accommodate the Cast Performance LBT 325, 335, and 360 grain bullets at 1.68" OAL, where the factory mid frame 45 colt cylinders are limited to about 1.65". Hodgdons starting loads for these bullets with H110 run from 18k cup to 22,400 cup. Knowing what these numbers are in PSI would be really helpful. Thanks!

[Paden]

An interesting read on the subject here: www.shootingsoftware.com/ftp/psicuparticle2.pdf

I have a fair bit of accumulated published 45 Colt data tabulated for comparison and extrapolation purposes, and it seems to indicate that in the cartridge in question CUP and PSI numbers tend to be relatively similar; probably well within the typical margins of variability and error (i.e. 23,000 PSI might equate to something like 25,000 CUP, but either of those figures might show a variability of something like +/- 1,000).

[500fksjr]

I have in the past called Mike at Cast Preformance an he has been able to run those #s an answer your questions....fwiw

[seancass]

Paden probably hit the nail on the head. In your gun with your components, the 15,000psi load is probably somewhere between 13 and 16k. So unless you are working in a lab, they're nearly equivalent or approximate.

Interested in what your application is that requires an answer more precise than 23,000psi is roughly 24-25,000cup. Are you researching new load data?

Edit: i reread your post more carefully. Looks like the starting loads are well inside safe limits, but 22k cup is probably a bit above 23k psi.

Now, why 23,000 psi? If i recall correctly, this number comes from a very well respected author's logic. The gun comes in 45acp +p so that's the safe upper limit, aka 23kpsi. This isn't necessarily a hard limit, it was arrived at thru logic. If 45acp +P was limited to 30k psi would these guns still be rated to +P? Only Ruger knows and they aren't saying. So, it's possible higher power loads are perfectly safe, but we don't know. And i certainly don't recommend you be the one to find out.

[singleaction]

May 25, 2016 7:12:44 GMT -5 seancass said:

Paden probably hit the nail on the head. In your gun with your components, the 15,000psi load is probably somewhere between 13 and 16k. So unless you are working in a lab, they're nearly equivalent or approximate.

Interested in what your application is that requires an answer more precise than 23,000psi is roughly 24-25,000cup. Are you researching new load data?

Edit: i reread your post more carefully. Looks like the starting loads are well inside safe limits, but 22k cup is probably a bit above 23k psi.

Now, why 23,000 psi? If i recall correctly, this number comes from a very well respected author's logic. The gun comes in 45acp +p so that's the safe upper limit, aka 23kpsi. This isn't necessarily a hard limit, it was arrived at thru logic. If 45acp +P was limited to 30k psi would these guns still be rated to +P? Only Ruger knows and they aren't saying. So, it's possible higher power loads are perfectly safe, but we don't know. And i certainly don't recommend you be the one to find out.

When working at the redline with a revolver, I think some certainty, based on the best available information, is prudent. I was hoping that some lab testing comparing both CUP and psi in the 45 Colt exists. This would put my mind most at ease. I'm curious what exactly makes you think 22k cup is probably a bit above 23k psi? 

I personaly view 23k psi as a hard limit for the 45 colt in this cylinder. Ruger confirmed to Brian Pierce that 23k psi loads (i.e. +P 45acp) are safe in the mid frame 45 Colt. I will not play beyond that boundary, period. Thanks to all for your comments!

[Paden]

Here's a small snippet of my collected data:

~285 grain cast:
16.0 2400 987 <14,000 psi Pearce HL #246
17.0 2400 1012 <20.000 psi Pearce HL #246
18.0 2400 1038 <23,000 psi Pearce HL #275
18.0 2400 1093 <20,000 psi Pearce HL #246
19.0 2400 1145 <23,000 psi Pearce HL #246
19.0 2400 1165 24,800 cup Linebaugh
20.0 2400 1195 <32,000 cup Pearce HL #246
20.5 2400 1294 29,800 cup Linebaugh
21.0 2400 1262 <32,000 cup Pearce HL #246
~310 grain cast:
17.0 2400 1013 24,400 cup Linebaugh
19.0 2400 1172 29,400 cup Linebaugh
~330 grain cast:
19.0 2400 1120 27,700 cup Hodgdon


What it tells me is that measuring PSI or CUP (or velocity) is probably anything but exact. As such, I personally don't care to work "at the redline with a revolver"...because the only way to tell exactly where the redline really is for a given cylinder is to test it to destruction, and you can only do that once.

While it would indeed be cool if someone had done a head-to-head comparison of PSI and CUP in a given .45 Colt chamber, under controlled lab conditions, I seriously doubt it has been or likely ever will be done.

[Rimfire69]

Correlating PSI and CUP, Denton Bramwell. Not sure if this is what your after, but I found it interesting.

[seancass]

May 25, 2016 18:15:34 GMT -5 singleaction said:

When working at the redline with a revolver, I think some certainty, based on the best available information, is prudent. I was hoping that some lab testing comparing both CUP and psi in the 45 Colt exists. This would put my mind most at ease. I'm curious what exactly makes you think 22k cup is probably a bit above 23k psi? 

I personaly view 23k psi as a hard limit for the 45 colt in this cylinder. Ruger confirmed to Brian Pierce that 23k psi loads (i.e. +P 45acp) are safe in the mid frame 45 Colt. I will not play beyond that boundary, period. Thanks to all for your comments!

It looks like you're correctly following best practices for reloading! Let me try to explain my wanderings. I have not personally seen the CUP/psi comparison. My exact reason for saying 22kcup is above 23kpsi is because 14kpsi is equivalent to 15.9kCUP. Most data I have seen puts CUP at about 10% higher number than psi(as these equivalent numbers for 45 Colt show). Looking at a given list of max loads for whatever cartridge from a variety of sources the max loads will list pressures at, for example, 33,000psi or 38,000cup. One assumes that both labs had the same max pressure, measured with different systems. If you had a bunch of time and a big reloading book(s), you could make a nice excel spreadsheet with equivalent psi and cup max pressures, then use that data to give you an extremely good idea where 23k psi lands. If I wanted a good answer to your question and google couldn't find the scientific answer, that's the backdoor method I'd use.

[chaz]

You really can't compare them, CUP Copper Units of Pressure had a small pellet of copper that compressed. Copper only compresses so far that is why SAMMI had to go to PSI. So at low pressures you can kinda do this but past a certain point it don't work.

[Paden]

May 27, 2016 12:52:23 GMT -5 chaz said:

You really can't compare them...

Sure you can. Bramwell clearly demonstrates it. (See linked paper in post #2).

...that is why SAMMI had to go to PSI.

SAAMI began transitioning to PSI after microelectronic technology made it possible. Not so long ago the copper crusher method represented the state of the art.

[minuteman]

Looking at ANSI/SAAMI Z299.3-2015, page 109 states...

"The designation “Copper Units of Pressure” (“CUP”) was adopted in 1969, to replace the previous designation of “pounds per square inch.” Advances in the art of pressure-sensing devices had shown that pressures recorded by deformation of copper crusher cylinders are not necessarily a true measure of pounds per square inch for the transient phenomena encountered in sporting arms ammunition."

I've always viewed PSI as the caliper and CUP as the micrometer.

On page 105 they give this reference...

VELOCITY: MEAN = Same as Corrected Average
HIGH = MEAN + 35 fps
LOW = MEAN - 35 fps
PRESSURE: MEAN = Same as Corrected Average
HIGH = MEAN + 2,500 CUP/psi
LOW = MEAN – 2,500 CUP/psi

Much of that manual is over my head, but the world of averages seem to apply everywhere. Hope it helps.