Written by Sierra Bullets Ballistic Technician Philip Mahin
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. It consists of individual cartridges, 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 pamphlet.
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. I have included a photo of a case (to the left) that was used in this method and you can see the ring left by the piston hole. What the book lists as the preferred method is the PSI 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. As far as I can tell and anyone else can tell me is that there is no correlation between them. An example of this is the 223 Remington cartridges 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 and you can see from at least one powder manufacturer that the 30-06 used both.
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. We use actual firearms to test data with so an example of our listed start load in the 308 Winchester using our 168gr MatchKing and Reloder-15 powder is 38.8gr or just round up to 39 grains. We found our accuracy at 42 grains with this combination and pressure signs at 43.6 grains. If you are to use the 1% rule and work up 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 from the 800 room every day. Till next time, be safe and enjoy your shooting.