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September 27th, 2015

CUP vs. PSI — Do You Understand the Difference?

SAAMI CUP PSI Cartridge Copper Units Pressure PSI
Image by ModernArms, Creative Common License.

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.

SAAMI CUP PSI Cartridge Copper Units Pressure PSICUP 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.

SAAMI CUP PSI Cartridge Copper Units Pressure PSI

Permalink Reloading, Tech Tip 1 Comment »
April 12th, 2009

Baney Tests Multiple Powders in 6-6.5×47

Asst. Editor Jason Baney recently commenced load testing with our 6-6.5×47 Lapua project rifle. (Built as a dual-caliber, switch-barrel test bed, this gun also has a 6.5×47 barrel). Our goal at this stage is to find basic starting points and max loads for a variety of popular powders. This is important because the 6-6.5×47 is a wildcat cartridge and very little “official” load data is currently available from the powder makers. We initially hope to give our readers a starting point when loading for the 105-108gr class of BT match bullets. Later we’ll work up load points for both heavier and lighter bullets.

Baney 6-6.5x47 Lapua Benchrest

Thus far, Jason has tested 6 powders: Alliant Reloder 17, Hodgdon Hybrid 100V, Hodgdon H4350, Norma N204, and Vihtavuori N160 and N550. With these powders, Jason shot ladders with charges in 0.5 grain intervals. All initial testing was done with Berger 105gr VLD bullets. Jason will continue testing next week with other bullets, including 115gr DTAC, 107gr Sierra MK, 105gr Hornady A-Max, 95gr Berger VLD, and 80gr Berger FBHP. He will also try some faster powders (Varget and Reloder 15) with the lighter bullets.

Alliant Reloder 17Reloder 17 Very Impressive and H4350 Performed Well
Thus far, the most promising powders, in terms of velocity, accuracy, and vertical dispersion, have been RL17 and H4350. According to Jason, “N204 was also very intriguing, displaying tight vertical, but it showed a lot of horizontal–though it was windy during that testing phase”. Shooting ladder tests at 200 yards, RL17 exhibited very tight vertical over a wide velocity spread, and 4 out of 6 loads went into a 1/2″ group at 200. Max RL17 velocity in our 30″ Bartlein barrel was 3199 fps. H4350 topped out above 3150 fps, but some pressure signs appeared at about 3120 fps. To top 3100 fps with H100V, Jason needed a compressed load. Velocities with N160 were quite slow compared to the other powders, and N550 maxed out at 3095 fps.

CAUTION: Jason was shooting with a 30″ barrel and a strong, custom BAT action. You may not be able to achieve the stated velocities in your barrel. When we’ve completed testing we will publish suggested starting loads for these powders.

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