Comments on: CUP vs. PSI — What’s The Difference in Pressure Measurements

By: Chris Callahan

Chris Callahan — Tue, 26 Apr 2022 01:12:21 +0000

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).

By: nfngun

nfngun — Fri, 16 Oct 2020 16:44:00 +0000

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.

By: Harry

Harry — Sun, 18 Aug 2019 14:36:49 +0000

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.

By: lil shot

lil shot — Tue, 06 Aug 2019 19:37:51 +0000

So Mark Hamel did you get it all converted in grad school?

By: Joe

Joe — Sun, 25 Nov 2018 23:27:50 +0000

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)

By: Mark Hamel

Mark Hamel — Wed, 26 Jul 2017 15:53:58 +0000

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.

By: Neil Gibson

Neil Gibson — Wed, 15 Oct 2014 07:56:45 +0000

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.