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Old May 5, 2012, 09:15 AM   #5
SL1
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Join Date: November 8, 2007
Posts: 2,001
About the only pressure sign that you can use at all would be the diameter of the pressure ring on the case after firing. That is NOT the same as case head expansion. It is the widest part of the case body, on the case wall above the case head. You need to measure that with a MICROMETER, not a caliper, so that you can get measurements in the ten-thousandths of an inch.

The problem with the method is that it is only a relative measure. The pressure ring depends not just on the pressure of the load, but also on the springiness of the brass, the exact wall thickness of each case, and the size of your particular chamber. And, it will reach a value somewhere in the neighborhood of 45,000 psi (depending on case springiness, etc.) where it will increase at a slower rate than at lower pressures (or maybe even reach a maximum value). So you cannot just keep shooting hotter loads because the pressure ring diameter is not increasing very much.

I have seen some gun writers suggest that you can approximate the pressure of a factory load by measuring the pressure rings of its cases when fired in your gun, then loading your own bullets, primers and powder into the same cases and firing them in the same gun, working-up the powder charge until the pressure ring gets to the factory value. To that I say "maybe."

The real issue is whether the first firing of the case changes its springiness enough to change the relationship between pressure and ring diameter on the next firing. In some "hard" brass cases like Winchesters, maybe not. In some "soft" brass cases like Federal, maybe so. The only way to know about a particular lot of factory cases is to pull bullets from 10 of the factory loads, deprime those cases, and reload each caseTWICE with a load that you can reproduce exactly. If they each produce about the same pressure ring diameter on their first and second firing, then you can use the pressure ring diameter you get from firing the other factgory loads as a goal for your own load work-up. if you cannot get the same results on successive firings, then you cannot rely on this procedure.

Also, remember that the pressure ring is dependent on both the INDIVIDUAL case, and POSITION ON THE CASE. So, you need to keep track of each individual case when you use this method and not compare rings on different cases for different firings. Also, as you measure pressure rings, you will notice that rotating the case in the micrometer anvils will give different readings, maybe varying by as much as 0.001" when you are worrying about 0.0001" differences. I don't think you can get a reproduceable "average" value because it is hard to reproduce the exact position of each "sample" reading of the case diameter. What I have settled on is to rotate the each case and take multiple readings, noting the highest and lowest. I then take the average of the highest and lowest values as the diameter for that case. You will notice that some cases are almost round, while others have a "bulge" area or a "dimple" area. And, the average of the high and low values for each case will differ substantially from case-to-case. So, you really need to pay attention to decide what amount of difference in you loads you can detect with this method. It is easy to fool yourself into believing something that you want to be true. (I also "index" my cartidges in my chambers so that the head stamp is always in the same orientation to guard against the possibility that the chamber is not exactly round, and only use one chamber of a revolver to test-fire cases because there WILL be noticeable differences between chambers in many guns.)

Also, when working-up a load, you need to consider whether repeated firings beyond the first two are changing the brass properties enough to mislead you. I do that by ALSO firing the SAME test load in the first ten cases as many times as I need to fire the other cases to work-up my load.

And, then there is the effect of the temperature at which the loads are fired. You need to try to keep your test firings at about the same temperature, because you WILL see a temperature effect in the pressure ring diameters if you change the temperature by too much. (No, I don't really have a recommendation on how much is "too much.")

Using this method can be quite tedious and requires a lot of cases. If I wanted to know the actual pressure of a favorite load, I would probably send it to White for real pressure testing.

On the other hand, I usually do keep track of pressure ring data when working-up loads for handguns and low-pressure rifle rounds. It is educational and does provide the first indication that something is different than expected. In that sense, if you have some experience with the technique, it is the first good "pressure sign" because it can be used all the way down to the old .45 Colt pressures.

SL1

Last edited by SL1; May 5, 2012 at 09:23 AM.
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