Estimating pressure via the pressure ring.


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KP95DAO
January 16, 2003, 09:18 PM
The use of case expansion to measure RELATIVE pressure is not a new thing. I ran across it in Col. Earl Naramore’s book, The Principles and Practice of Loading Ammunition. It is well worth the money if you can find a copy. Mine was printed in 1962. His discussion of the topic is contained in Chapter 48 on page 827. It ends on page 842. As you can see it is lengthy. It can be boiled down so that it is usable for the diligent reloader. So here is how I use this method.

THIS METHOD SHOULD ONLY BE USED FOR CARTRIDGES AND FIREARMS WHICH ARE DESIGNED FOR PRESSURES ABOVE 20,000 PSI.

The case body joins the case head with a fairly sharp increase in the thickness of brass. As the pressure increases, the expansion of the brass to fill the chamber comes closer to the base of the cartridge. As the chamber is larger toward the breechface, the closer the expansion is to the base the larger the diameter of the expanded case. This “pressure ring” (the largest diameter of the case ahead of the extractor groove) is consistent in its relationship to the pressure involved if all other factors (case, bullet, primer, powder type, seating depth, etc.) are the same. The only variable should be powder charge weight.

The measurement of this ring can be achieved by the use of a caliper or a micrometer. I use a caliper that has a .2” range with one revolution of the needle. For those with a .1” per revolution it will be even easier to read an increase. I record differences of .00025”. This measurement has to be over the point of greatest expansion. You need to rotate the case while measuring to find this point. For some firearms, my NEF single shots for instance, it always occurs on the top and bottom of the cartridge in relative to the chamber. I always index my rifle rounds during reloading procedures and when firing. For my Glock 32 it is greatest on the sides of the case relative to the chamber. Again, we are talking about a couple of thousandths; but, they are important. So make sure you are measuring the largest part of the ring. And make sure you are not measuring a ding or burr on the case.

You can use factory ammo to establish a baseline. I use a chronograph when working up loads so I can tell when I am getting close to the desired velocity. The pressure ring measurements, taken off the workup loads, provides a clue as to where you are pressure wise. If you go until you start getting another obvious high pressure sign, sticky extraction for example, you will have charted the pressure rise with the Pressure ring and will then be able to apply that knowledge over to another load—USING THE SAME TYPE CASE.

Of course any change will affect the pressure. With this method you will be able to tell where you are for that case and firearm. Generally you will not be loading at the level where changing cases will push pressures above what is safe. At least if you’re smart you won’t. If you want to change cases, work up again and take your reading. I have found that MOST BRASS will give the same readings or at least close enough to count providing capacity is the same. There are some case brands which are weaker. These are found primarily in handgun rounds. As long as you use Fed, Rem, Win, Starline, PMC, Frontier, Hornady, US Military, brass you will be all right. And there are some companies who have their brass made by the above mentioned companies which are fine.

These reading are good for that chamber only. So each rifle will have to have it’s own readings. Likewise a revolver has a number of chambers and each chamber will be different. So what you do in that case is to pick one chamber, I use the one with the smoothest walls and throat. This chamber will be your pressure test chamber. I also use that chamber in my wheel guns to check for load accuracy. I load up until I encounter sticky extraction and then back off to some level below that. An extremely strong gun (Redhawk 357) might not give sticky extraction before encountering primer pocket expansion. You may find that a load which is not sticky in that chamber will be somewhat sticky in another chamber. This will be as a result of the roughness of the walls and not as a consequence of increased pressure unless the other chambers are tighter or have smaller and or rougher throats. Again, unless you are loading on the edge, this should not be a problem.

Auto loading pistols are handled like a rifle except you don’t have sticky extraction to be your guide. You pretty much have to use a factory round to establish the pressure level. One other good way is to increase the load until you start to get casehead expansion which will lead to primer pocket enlargement over one or more firings depending how much over the “safe” level you are. You can then use the pressure ring measurements to keep away from the primer pocket expanding loads.

The bottom line is that this is just another tool in the reloader’s tool box. Its effectiveness will depend upon the reloader’s correct use and proper understanding of the information it will deliver. GUESS WHAT? THAT PUTS IT SQUARE ON YOUR BACK TO USE IT CORRECTLY AND TO UNDERSTAND WHAT IT IS TELLING YOU.

As I have stated before, I have used this method for over twenty years without an unexpected pressure reading. I strongly recommend the Col’s book. Not just for this info; but, for the wealth of other, if dated, information.

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WESHOOT2
January 18, 2003, 12:48 PM
Thank you for posting this.

Cal4D4
January 19, 2003, 12:07 AM
Geez WESHOOT2... a new disciple of SAAMI? Naw, couldn't be!:D

WESHOOT2
January 19, 2003, 11:01 AM
Nope.

Disciple of safety.
Screw SAAMI :neener: .

Hahahahahahahahahahahahahahahahahahahahahaha ha!

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