October 13th, 2014

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.

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

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 2 Comments »
October 13th, 2014

Anette Wachter Helps Lead Fight vs. I-594 in Washington State

Story based on report by Lars Dalseide for NRABlog.com
I first met Anette Wachter four years ago at the NRA’s National Long Range High Power Rifle Championship in Camp Perry, Ohio. A statuesque brunette, Wachter easily stood out in the crowd. I just so happened to be a big fan of her twitter feed as well. After a quick exchange of pleasantries, we talked about shooting, life in Washington, and how she could get more involved.

Anette Wachter I-594 NRA Washington

Since then she has become very involved. Her 30 Cal Gal blog has become a favorite of male and female shooters alike. Due to her diligence, she was also about to earn a spot on the U.S. National Palma Travel Rifle Team. And recently, Wachter lent her voice and views to the battle for gun rights in the state of Washington.


by AccurateShooter.com

Anette Wachter, aka “30 Cal Gal”, Helps Lead Fight Against I-594
Below is a video Anette made opposing I-594, a Washington State initiative on the November ballot. This law could have drastic consequences for Washington state firearms owners. Visit www.VoteNo594.com for more information and to learn how you can help defeat this gun control initiative.

The provisions of I-594 are explained in detail on the NRA-ILA website. Here are the key reasons Washington gun owners should oppose this proposed legislation:

Anette Wachter I-594 NRA Washington

Permalink - Videos, News 1 Comment »