Originally Posted by denton
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Here is some actual data from a Finnish Mosin instrumented with a strain gauge. The standard deviation of the random error in that particular system has been measured at 335 PSI, which is practically the same as a good piezoelectric system. The case head measurements were made with a Mitotoyu blade micrometer, clamped to a granite inspection block to ensure that all measurements were made at the same height on the case head. Since cartridge cases are not perfectly round, three measurements were made on each case at different orientations, and the three measurements were averaged.

The brass was all from the same batch, having been fired once.

Note that there are two rounds that were over 60 KPSI that measured .0001" or less of case head expansion. If you follow the 95% PI intervals across the chart, .0003" represents something between 49 KPSI and 75 KPSI. Close enough for you?

The R-Sq value shows that 23.5% of the change in case head expansion is attributable to pressure, and the remaining 76.5% is attributable to random error. If you like a measurement system that is 3/4 random error, then CHE is for you.

No matter what you do, case head expansion is NOT a reliable pressure indicator. If it were, that's what industry would use.

You will not look better with a rifle bolt where your nose once was.


So out of curiosity, did anyone section the cases, and run some microhardness measurements on the brass, to determine the hardness range in the case webs? It would also be worthwhile to measure the internal dimensions of the brass, to see if is consistent. You mention it's a Mosin, so 7.62x54R. I never have worked with one, but I would wonder how consistent the brass is made for them - quality or plinking grade.

Measurement error is a fickle witch, and I've run into it many times over the years in manufacturing. The one I always remember is some stampings that we heat treated, which had to be tested at a GM-approved lab for hardness, to be approved as first articles. The lab significantly disagreed with our testing. After a week of head scratching and phone calls, I sent the same specimen to a different GM approved lab. They agreed perfectly with us. So I submitted the first articles with their test report smile

I agree, you can't say .0002" expansion is 50ksi, .0004" is 60ksi, etc. Too many variables are in play. But I'm still going to measure case heads, because if they're expanding too much, there's a problem, regardless of what the chrono or other instrument is telling you.

"...the designer of the .270 Ingwe cartridge!..."