CUP vs. PSI — What’s The Difference in Pressure Measurements
by Philip Mahin, Sierra Bullets Ballistic Technician
This article first appeared in the Sierra Bullets Blog
The ANSI / SAAMI group, short for “American National Standard Institute” and “Sporting Arms and Ammunition Manufacturers’ Institute”, have made available some time back the voluntary industry performance standards for pressure and velocity of centerfire rifle sporting ammunition for the use of commercial manufacturers. [These standards for] individual cartridges [include] the velocity on the basis of the nominal mean velocity from each, the maximum average pressure (MAP) for each, and cartridge and chamber drawings with dimensions included. The cartridge drawings can be seen by searching the internet and using the phrase ‘308 SAAMI’ will get you the .308 Winchester in PDF form. What I really wanted to discuss today was the differences between the two accepted methods of obtaining pressure listings. The Pounds per Square Inch (PSI) and the older Copper Units of Pressure (CUP) version can both be found in the PDF pamphlet.
Image by ModernArms, Creative Common License.
CUP Pressure Measurement
The CUP system uses a copper crush cylinder which is compressed by a piston fitted to a piston hole into the chamber of the test barrel. Pressure generated by the burning propellant causes the piston to move and compress the copper cylinder. This will give it a specific measurable size that can be compared to a set standard. At right is a photo of a case that was used in this method and you can see the ring left by the piston hole.
PSI Pressure Measurement
What the book lists as the preferred method is the PSI (pounds per square inch or, more accurately, pound-force per square inch) version using a piezoelectric transducer system with the transducer flush mounted in the chamber of the test barrel. Pressure developed by the burning propellant pushes on the transducer through the case wall causing it to deflect and make a measurable electric charge.
Q: Is there a standardized correlation or mathematical conversion ratio between CUP and PSI values?
Mahin: As far as I can tell (and anyone else can tell me) … there is no [standard conversion ratio or] correlation between them. An example of this is the .223 Remington cartridge that lists a MAP of 52,000 CUP / 55,000 PSI but a .308 Winchester lists a 52,000 CUP / 62,000 PSI and a 30-30 lists a 38,000 CUP / 42,000 PSI. It leaves me scratching my head also but it is what it is. The two different methods will show up in listed powder data[.]
So the question on most of your minds is what does my favorite pet load give for pressure? The truth is the only way to know for sure is to get the specialized equipment and test your own components but this is going to be way out of reach for the average shooter, myself included. The reality is that as long as you are using printed data and working up from a safe start load within it, you should be under the listed MAP and have no reason for concern. Being specific in your components and going to the load data representing the bullet from a specific cartridge will help get you safe accuracy. [With a .308 Winchester] if you are to use the 1% rule and work up [from a starting load] in 0.4 grain increments, you should be able to find an accuracy load that will suit your needs without seeing pressure signs doing it. This is a key to component longevity and is the same thing we advise [via our customer service lines] every day. Till next time, be safe and enjoy your shooting.
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Tags: Cup, Pressure, PSI, Reloading, Sierra Bullets, Transducer
I’d guess the discrepancy it’s a combination of two main factors.
The first, going by SAAMI diagrams, is that the position of the crusher gauge port varies with cartridge. From testing tank guns and artillery, the pressure within the chamber varies with the chamber geometry and distance from the base. In general, the nearer the base, the higher the recorded pressure nears the true peak pressure. Without measuring each cartridge at the base, you’ll never read the correct peak pressure.
The other fact, which I think is the main problem, is that the deformation of the crusher gauge is a record of the total force on the gauge over the entire internal ballistic cycle, not just a measure of the peak. Cartridges may have the same peak pressure, but different cycle times, so the total force and hence gauge deformation varies.
This really makes me mad. Has no physicist or engineer ever put in the time to be able to do a stupid unit conversion? With every other set of units in the world it’s brain dead simple but of course the reloading units manage to get totally screwed up? I know what I’m gonna solve when I get to grad school.
There is no direct ratio to convert psi to cup because of the way the measurements differ. One: psi is a direct measurement, cup is a derived measurement. And two: with the way cup is derived, doubling the cup number does not (unlike psi) double the actual pressure. One copper unit of pressure is some amount of deformation of a copper pellet which through testing is equated to some pressure in psi. However, the pressure need to double that deformation is more than double the pressure in psi. And just as an aside, psi and cup are equal for .38 special (at least I’m pretty sure that is the cartridge. I know they are equal for one)
So Mark Hamel did you get it all converted in grad school?
I am trying to determine which post WWII ctg can be used in the Swedish Mod 96/38 Mauser action. These are stronger than the 93/95 Mauser actions as they were made with a higher grade of steel furnished to the Germans by Sweden. Info indicates thes Swedish small ring actions were proofed at 66,000 PSI. However, most of the current load data uses CUP to determine pressure.
Note: The original 6.5×55 was introduced in the earlier Krag rifle, so the Sweds did not load the ctg to its full potential when they went to the Mauser 96/38 rifles actions.
Very simple solution to all of this! Someone just needs to test both ways.
223/556 being the biggest concern. Its not that hard to test both .223 and 5.56 ammo in the 3 most popular chambers using both methods. To end the mystery.
SAAMI reports the results in both CUP and PSI in their standards. It appears that they use a different CUP testing rig depending on the round.
https://saami.org/wp-content/uploads/2019/02/ANSI-SAAMI-Z299.4-CFR-Approved-2015-12-14-Posting-Copy.pdf
e.g.
For 223 Remington PSI = 1.058 x CUP
based on MAP = 520,000 CUP (p.14) and 550,000 PSI and (p. 26).