Home Forums Hunting & Shooting Ask The Gunwriters Measuring case pressure with a micrometer

I just saw this release for measuring pressure with a .0001" blade micrometer.

So, I picked up a good used one on eBay. Made the seller an offer well below ask and he took it.

I'd rather have a good quality used tool than a new Chicom clone. This one is a Mitutoyo, a well known and respected Japanese brand.

Check it out.

DF

That's a fine instrument. It should serve very well.
One important consideration with respect to measuring CHE is that "absolute" accuracy is not particularly important. IOW, if your mic's measurement of the unfired dimension of your case is (consistently) .4669" (for example), it isn't terribly important that it might actually be .4665 or .4671. What's important is that when you go back to take the post-firing measurement at the same spot on the case, the relative measurement is accurate and repeatable. Hopefully even a Chicom clone can do this.
Looks like you have a very nice blade mic there. My blade mic is a Chicom but my "standard" mic is a Lufkin, and it's the nicest mic I ever used.

Regards,
Rex

This is a chart of change in case head diameter (CHE) on the X axis and measured pressure on the Y axis. CHE data were taken with a very fine Mitotoyu digital blade micrometer, clamped to a .0005" flat granite inspection plate.

The graph tells a story.

It says that a table of random numbers will give you almost as good an estimate of pressure as a good micrometer will.

If CHE really worked well, reloading labs would use that instead of expensive piezo or strain gauge equipment.

Sorry to harsh your mellow, but CHE is a very poor measure of pressure.

Just read an article by Rick Jamison about this. Bottom line after all the fussing was that he doesn’t feel like the results he gets are particularly accurate and it’s a lot of damn trouble.

Staying inside the lines seems like the best way for me at least. There’s plenty of properly tested data and to spare available for me to do what I need to do.

That and a decent chronograph.

Well, regardless how well it works, as a gadget guy, I took the bait.

It is interesting.

DF

I took that bait years ago, based on the claims of the very highly respected writer Ken Waters. After measuring and tabulating something like a gazillion unfired and fired cases, I can confirm one indisputable fact: fired cases are larger than unfired ones.

And that is ALL you can learn from the CHE method.

Same here Rocky.

Interesting is always a good reason, and a feller can never have too many gadgets. Enjoy.

I don't believe I could get consistently accurate-to-a-tenth readings of a tapered, tubular, brass case head that's been fired in a tapered chamber with my own (US made) Starrett micrometers or my (Swiss made) Etalon micrometer that have been in my possession since they were factory new, have been professionally maintained, have never been dropped or over-stressed in use, and are periodically checked for accuracy with gage blocks at several places from 0 -1.0000" and at various rotational orientations of the spindle. I would be more than a little skeptical of readings acquired with a Japanese blade micrometer of unknown provenance.

I honestly believe piezo or strain-gage equipment is the only reliable way to obtain chamber pressure readings. I don't buy into the use of chronographs as pressure indicators either. If you compare the speed of your load with a particular bullet to a factory load with the same bullet, I guess you could get a pretty good idea that your load is or isn't in the ballpark with the factory load, but that's about it. It just seems to me that there are other factors including actual bullet diameter, actual bore measurements and the condition of the bore that can slow a bullet down and at the same time, increase pressure. Why wouldn't it be possible to have a tight spot ahead of the chamber that would create a great spike in the pressure curve at that point, that would be dissipated by the time the bullet reached the end of the bore and didn't affect exit speed?

I've played with CHE. When developing loads I shoot over a chronograph, but I also mind such things as primer condition, bolt lift and the appearance of the case head.

It's all good fun.

Yeah, I know, I have the same mic as yours, lol.

15 years ago on various forums CHE was the source of some white hot flaming threads.
Ask Denton about it........

I agree, but even though it won’t provide hard data, I think the chrono data is a worthwhile tool that can help keep us from disaster, maybe before the other seat-of-the-pants tools show anything.

Still don’t have one, but have finally settled on the Magneto Speed as the best overall unit. Just have to let the rifle-buying dust settle before I order it.

Magnetospeed set up free standing is about optimal, IMO. And, I've had the optical types. Does best with a longer bench. I built this one for my Z-71 tailgate.

There have been threads on this topic, these posted before. JB set up one sorta like this.

DF

Hasn't Ken Waters or someone else tested this against lab pressure or strain gauge results? I don't ignore the Voodoo but as others have posted I trust the chronograph and published data more than anything else. To many variables like brass hardness and chamber dimensions at work for me to trust case head or web measurements, combined with my lack of ability to consistently measure beyond three or four decimal points.

No, your brain & common sense are the best way to avoid disaster........................

Old timer that didn't all have chrono's did not blow up much stuff, but they may have been on the edge.

Being on the edge & going over are two different things.

Most current handload data is pretty reliable; it's the people that think they know better that usually get themselves in over their heads.

And those who think that that extra 50 FPS from their 300 grain superbullet from their new superwhoozit magnum is going to change the world.

MM

Almost 18 years ago I ran a test using a very good micrometer on three different cartridges, then had the same handloads pressure-tested in a piezo laboratory. On one cartridge the technique was very close, the second turned out to be way underloaded, and the third turned out to be way overloaded.

The the handloads' velocities turned out to be a much better indicator.

Wrote this all up in an article for Handloader. I haven't bothered to mike a case since then.

That Jamison article would be a good thing to be included in any discussion of CHE.

That was my motivation to give it a try and my discovery as well.

So, on average, the technique was good to go huh? 😊

Nah, just always test the first of the three.