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

That’s an interesting point.

Still, I’ve had different pieces of brass, from the same lot, identical loads, shot one after another, vary a fair amount when measuring CHE.

I guess if a guy used a number of cases, then took some kind of average. Still, a guy needs to know what the max CHE is that indicates max pressure for his load, including that particular lot of brass. In our case, short of a pressure gun, we are back to looking at the chrono and velocities.

Yes, there are a number of possible contributing factors there. One is of course some degree of variation in YS and thickness between cases, though you'd expect that to be fairly small in cases from the same lot. Another is the repeatability of your measurements. For example, are you measureing between exactly the same points on the case (or, better, between flats produced with a fine file), using the same clamping tension, and how good is your micrometer? The other thing which comes into play with brass is that when it yields stress vs strain becomes distinctly non-proportional - there's a sudden increase in plastic deformation without increase in applied stress in the area of the stress/strain curve just past the yield.

This is another reason why we don't see a linear relationship between chamber pressure and CHE. What instead you have is, as I said earlier, a much simpler set of criteria: if there's some small but measurable amount of CHE, then you are pushing it. If it is enough to do things like make primer pockets loose and/or cause plastic flow into the ejector then that is well over what that particular lot of brass should be expected to take.

I have a Mitutoyo blade mic. I bought it a long time ago when playing with a Oehler M43 Ballistics Lab.




Another good point. But...what comes first? CHE that is indicating overpressure or the obvious signs of over pressure as in ejector marks and flattened primer?



Are you saying that measuring a fired case (not a brand new case but one that has already been fired one or more times) should no longer show any expansion above the web?

The first person I've seen credited with popularizing CHE was Bob Hagel. I was a poor college kid that was hell bent on quantifying everything. My dad wasn't thrilled when I told him I wanted a ten thou micrometer. I wasn't thrilled when the practicalities of measuring a cylinder looking for a couple of tenths difference showed me the follly of my expenditure.

A few decades later I got a strain pressure setup and did a comparison for old time sake and found that not only is there not a linear relationship between CHE or Web Expansion and strain measured pressure, but there wasn't even a binary correlation that I could demonstrate.

Well, if we are seeing plastic (ie permanent) deformation of the case head, it means we are exceeding the yield stress (YS) of the brass. Subject to what I said about that portion of the stress-strain curve just past the YS, where you tend to get a sudden increase in strain even as stress doesn't increase, in general the more deformation you are getting the more you are pushing things. As a fairly crude approximation then, a bit hot and the case head might expand enough to measure with your micrometer. A bit more and you'll start to see other signs, such as primer pockets being a bit loose and/or marks on the face of the casehead. To some degree the signs will also depend on the particular geometry in your particular rifle - factors like whether the case head is well supported in a tight chamber, for example.



That is an interesting question. Each time there's some plastic deformation of the case head, you have had to go past the YS of the brass to do it. The elastic portion of the strain is recovered but the plastic portion isn't. The brass undergoes a small degree of work-hardening and the YS increases very slightly. If you are continuing to get plastic deformation with every subsequent load you are potentially running further up the pressure curve each time to do it.

Now if the expansion is very small each time you may make an informed decision to tolerate it, knowing that sooner or later you'll find that the primers will be too loose to stay in place, and you'll be binning that brass. That is your call. Your alternatives include loading just a little more conservatively, and having brass which will last a lot longer, or going with a cartridge which will give you the bit extra you think you need.

That is interesting, because CHE is, quite literally, "strain measured pressure". So for that matter is a copper crusher. What strain was your measuring device measuring? Elastic strain on the barrel reinforce (Nock's form)?

Lame'...closed thin wall vessel. I'd have to go back and check the amount, but there was an offset to allow for the yield strength of the brass case. Off the top of my head it was 2 or 3kpsi, but I'd have to go back and look.

I don't know the answer, but my swag is that with CHE you are measuring brass that has exceeded elastic limits (permanent deformation) where in strain the expansion is mostly transient. Continuing the swag...the deformation you are looking for in CHE or are able to detect is not as consistent as with strain.

Hmmmm.....

I don't often engage in discussion of PRE and CHE anymore. But, what the heck....


Data taken with a digital blade micrometer, 50 millionths resolution.

Just look at the pressures represented by 0.0000" case head expansion. You have one case where .0.0000" corresponds with about 41 KPSI. You have another instance where it corresponds to about 62 KPS. Same dimension, over 20 KPSI difference in actual peak pressure.

At .0001", you have one cartridge that measured 49 KPSI, and another that measured 63 KPSI. Well, that's only a 14 KPSI range for the same CHE. So much better, right?

Then look to the far right. There are two instances just above the trend line with CHE of .00035" and .0004", and compare them with the 60+ KPSI cartridges on the left.

One critical number in the graph is S=7.84. That is the standard deviation of the random error in the model. That says that you can expect about 95% of cartridges to fall within 15.7 KPSI (2 X 7.84) of what the micrometer "thinks" is the right value. IOW, your measurement puts you somewhere within a 31.4 KPSI (i.e., plus or minus 15.7 KPSI) window most of the time. Or look at R^2. That says that the data contain about 1/4 information and 3/4 random error.

You can take a whole bunch of measurements and average them to get rid of much of the random error. But you're going to need a much bigger cartridge box.

To clear something up about terminology, strain simply means the change in dimension when a load is applied. When you apply, say, tensile stress to a piece of brass it gets longer, for example. That is strain. Within what is known as the elastic limit (ie up to the yield stress) that strain is directly proportional to the applied stress, and fully recovered when the stress is removed. IOW, the piece returns to its original length.

If you continue to apply more and more stress though, the brass "yields" - it starts to deform plastically. This plastic strain is not recovered when the stress is removed, and the dimension of the article is permanently changed. That is what you are seeing with something like case-head expansion: you have applied so much stress to the case head that it has exceeded the yield strength and been permanently deformed.

I am with you who say the velocity is the best estimator of the handloads pressure. I must add: with the correct powder, to me the fastest one for a 100 % load density for the cartridge-bullet-case combination. Coupled with ANY evident overload sign. What I do is a reverse thing: once reached my goal for an ESTIMATED maximum SAFE load, then I do some CHE measurement. Is an education itself! For an example, you can discover soft vs. normal or harder cases heads between brands and/or brass lots. I have discovered some factory loadings, specially some Sellier & Belliot .30-06 180 grs loadings, shows CHE of 0,001". With some other slightly over pressure signs...

All good points, patagonia hunter.

Personally, I really can't understand why this thread keeps stumbling along. tI has long been demonstrated that CHE has a poor correlation to chamber pressure, no matter the techniques and instruments used, partly due to vast differences in brass thickness and hardness in the same cartridge.

Which is I am really puzzled why the "information" in the first post is on their website. I know the technical staff at Hodgdon well, and they have expressed the opinion that even strain-gauge pressures aren't as accurate as piezo-electronic equipment, because of "too many layers" between the powder charge and the sensor. Dunno why they would suggest such EXACT amounts of case expansion as meaningful.

Comparative muzzle velocity, used correctly, has also proven to be a more accurate method of pressure "safety" than CFE, and adequately accurate chronographs can be purchased for about the same price as a very good micrometer.

But whatever....

Head expansion has many factors that include the pressure and the yield strength of the particular piece of brass measured.

Personally I am not really concerned with absolute pressure as the failure point is when the brass yields and the primer pocket expands or more scary the case head ruptures.

A skilled fellow with a mic can see when the brass is yielding before primer pockets get loose.

Brass will work harden after being fired so brand new brass is not a good way to test.

If your load is expanding the case head on once fired cases it's pretty hot for the brass you are using.

It almost feels like secret knowledge.