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What the heck is C.U. P. pressure

This is a discussion on What the heck is C.U. P. pressure within the Reloading forums, part of the Firearm Forum category; I have been trying to find out what CUP is and how it effects the weapon and the bullet....


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Old May 13th, 2013, 05:35 AM   #1
 
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What the heck is C.U. P. pressure

I have been trying to find out what CUP is and how it effects the weapon and the bullet.



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Old May 13th, 2013, 05:55 AM   #2
 
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It stands for copper units of pressure and is based on the use of a copper cylinder in a crusher gun, where a piston crushes a precisely made copper cylinder, that is then measured to determine the CUP.

It's not a linear comparison to psi, as a longer duration of the pressure pulse in a pressure gun can have the same effect as greater pressure with a shorter pulse, so there is no direct formula to convert from CUP to PSI. Some people try to approximate it but the caution here is that it's only linear over a very narrow range, so an approximation table or formula that may work for a 55,000 psi class cartridge, won't work for a 25,000 psi class cartridge. In general though, CUP pressures are lower than piezo electric measured PSI pressures.

Last edited by Model 52; May 13th, 2013 at 06:03 AM.
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Old May 13th, 2013, 05:56 AM   #3
 
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Technically speaking, "CUP Pressure" would be a redundancy similar to "ATM Machine" because the "P" in "CUP" stands for Pressure. So, correctly it would just be "CUP".

It stands for "Copper Units of Pressure" and is a method of estimating pressures inside the chamber of a firearm based on how much that pressure deforms a copper cylinder in comparison to deformations of other, identical cylinders under known pressures.

For example, if a force of 5 psi squishes a copper cylinder 10% in tests, then you fire a special gun that allows the chamber pressure to act on an identical cylinder which squishes by 11%, you can estimate that the chamber pressures were just slightly above 5 psi.

(Obviously I'm using ridiculous numbers for illustration purposes only).


There is also a unit known as LUP (or "Lead Units of Pressure") which is used in shotgun ballistics rather than copper because shotgun pressures are much lower than handgun or rifle pressures.

CUP or LUP is quickly being replaced by piezoelectric measurement techniques that give more direct, more accurate measurements.

The important thing to know about CUP is that when you read a chamber pressure given as CUP, then you know it's an ESTIMATE of pressure based on the squishing of copper cylinders, rather than a direct measurement.

How does it affect the weapon and the bullet? More pressure in the chamber means more stress on the metal of the receiver, obviously. Also, it means faster burning and/or higher amounts of powder. Usually (depending on barrel length and bullet shape) you'll also see higher muzzle velocities as chamber pressure goes up, which (again, depending, now on sectional density and bullet design and range) may result in greater impact energies.

Last edited by jpvanhoy; May 13th, 2013 at 05:58 AM. Reason: minor oopsie
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Old May 13th, 2013, 05:59 AM   #4
 
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CUP Versus PSI
this tells you ..... I think
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Old May 13th, 2013, 06:00 AM   #5
 
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Though I don't know where there would be a "chart" and I probably could not understand it anyway...I'm told that CUP gives a "final result" of pressure after the "event" takes place but does not show spikes in pressure or the pressure curve getting to that final result...so a CUP reading may give an indication of pressure it does not show how the pressure was generated. I'm told that has some bearing on how different weights and designs of bullets and powder burn rates react in the same caliber.
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Old May 13th, 2013, 06:26 AM   #6
 
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Here is a PSI vs CUP correlation PDF
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Old May 13th, 2013, 09:02 AM   #7
 
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Model 52 nailed it!
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Old May 13th, 2013, 10:09 PM   #8
 
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This is exactly why I love the whole forum concept!!!!
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Old May 14th, 2013, 01:11 AM   #9
 
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Quote:
Originally Posted by cwo2usn208 View Post
I have been trying to find out what CUP is and how it effects the weapon and the bullet.
Do you handload?

Pressure, be it CUP or PSI is basic stuff that should be understood. It is easily found and covered in any loading manual that I've owned.
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Old May 14th, 2013, 04:05 AM   #10
 
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I want to thank all of you for getting back to me, what prompted the question is my investigation of using cast bullets in my .270. Handloading with 18 grains Unique is possible, but the CUP is 55,000 where loading with an 40 grains of an IMR powder the CUP is around 35,000. I guess my question basicly comes down to will the Unique load potentially damage my weapon?
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Old May 14th, 2013, 05:17 AM   #11
 
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Quote:
Originally Posted by cwo2usn208 View Post
I want to thank all of you for getting back to me, what prompted the question is my investigation of using cast bullets in my .270. Handloading with 18 grains Unique is possible, but the CUP is 55,000 where loading with an 40 grains of an IMR powder the CUP is around 35,000. I guess my question basicly comes down to will the Unique load potentially damage my weapon?
The SAAMI spec for .270 Winchester is 65,000 psi.

As noted above CUP and piezo electric derived PSI are not linear, but based on the Bramwell formula of PSI = 1.52(CUP/1000) - 18, 55,000 CUP produces the PSI equivalent of 65,600 psi, just a bit past the 65,000 psi SAAMI specification.

Given that the formula is also designed for pressures between 28,000 and 54,000 CUP, and that it is at best an approximation, I'd back the load down a bit and work up.

Unique is used in many rifle cast bullet loads as it tolerates large empty spaces in the case well, so reducing the charge is probably not an issue, however I'd hesitate to use it at all without adequate data. Ideally, find some data for the .270 with the cast bullet of choice and start the load development in the middle.

----

To be honest, while I like Unique in my .375 Win, it is not my powder of choice for a .270 cast bullet load. IMR 4895, 3031 or 4198 all work well as does RL7. For a pistol powder RCBS indicates Red Dot can be used (11.5 grains to 13.5 grains) or 700x (11.0 gr to 13.0 gr) with its 270-130-FN and 270-150-SP bullets, and I've seen 2400 data from 16.5 gr to 20.0 gr of 2400 with a 230 grain NEI 130.277 bullet.

Last edited by Model 52; May 14th, 2013 at 05:28 AM.
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