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.

Like many here, I did a bunch of measurements for CHE expansion and like others the data varied so widely that even a simple guy like me dismissed it. Sometime later I read Denton's paper and he quantified my suspicions. I thought measuring CHE was dead and gone, then in the past several months I have read several articles claiming it is a valid technique. I will stick to my loading manuals and chronograph.

405wcf

Thanks all for the smiles!

Well, at least I got a very nice blade micrometer at a great price...

DF

I also thought measuring CHE was not a reliable measurement. I do remember Mule Deer's article and also him mentioning same. A while back I researched it again when I had primer cratering that turned out to be an oversize firing pin hole. Yes, it was a Remington 700. My son's rifle does the same thing no matter the charge. I had my bolt bushed by Greg Tannel.


Here is another article by Hodgdon for what it's worth:

https://www.hodgdonreloading.com/re...k-monitoring-pressure-your-rifle-reloads

There is always the bent paper clip!

Ken Waters published his method of developing loads by measuring base expansion in September 1982, but I'm pretty sure had been using it for some time prior to that date... possibly decdes prior. That's long before I-phones & the internet. At the time it was a viable method of judging relative pressure. Not perfect, but a whole lot better than a sharp stick in the eye. Same with p-ring expansion. The crusher method was what the more advanced ballistic labs were using at the time. I'm not even sure there were strain gauges back then, but doubt they were all that reliable if there were.

Time moved on & new methods were developed, then improved. I wonder what will replace today's hi-zoot transducers in another 25 or 30 years. I dont have access to that technology. Neither do most of you. We dont even have access to an outdated copper crusher set up. We do have access to micrometers, chronographs, & hopefully some sense. Yeah it's a pain, but sometimes there's not much else we can do.

Chronographs also have flies when used alone for developing max loads. An example is P/P2000MR in the 35 Whelen. Absolutely unheard if velocities within normal pressure boundaries have been published with this combination. If we were using only a chronograph for development, most of us would have stopped well below its potential.

A new rifle shows light ejector marks with rather anemic factory loads from a slight burr on the bolt face. the barrel is longer than most loading manuals use. It's in a chambering I've never used before so I have no reference. My new lot of 4350 if significantly slower than the old lot & most certainly different than what the manuals used. Where do I stop? What about my wildcat? There is no data at all. What about old cartridges in new rifles that are underloaded for older rifles? What about semi-obsolete cartridges that haven't had data reshot since the 70s?

They're all tools we have to work with. More data is usually better.

Mics are valuable pieces of equipment. You done good!

You didn't hear this from me, but when portable speed guns came out, British police departments had a lot of officers on sick leave. Despite not carrying handguns on the job, they were still trained on their use. Too many of them had this silly idea that they could measure the speed of bullets by using their radar guns. The trouble was, the officers were trained to have traffic travel toward them. They carried this training with them onto the range.

I won't bore you with the rest...

One thing, IIRC, Mr. Waters did not measure case head expansion but went with the pressure ring expansion instead. He felt it gave hime more reliable results.
Paul B.

This could be totally off base, but I felt that he used the pressure ring because he was using a standard micrometer and could not avoid the pressure ring (and rim, which could be filed down) and touch only the head.

The pressure ring is indeed a better indicator than the case head, but that's being the best pig of a bad litter.

Back in the bad old days, doing piezo and strain measurements was a real challenge. Harold Vaughn, of Rifle Accuracy Facts fame, hauled a Tektronix 555 oscilloscope (big, heavy, true dual beam) and a generator around in his pickup. Floyd Brownell used the Tektronix 564 with a differential amplifier plug-in for his major piece of work, Firearms Pressure Factors. Those instruments were vacuum tube based, and vacuum tubes tend to drift a lot. People were using "chopper stabilization" back in the day, to get around that. Doing good single event measurements took a ton of work.

In the 60s, electronics started to turn solid state. Suddenly, many of the problems of the past weren't problems anymore. Piezo and strain measurement became much more feasible.

Ken Oehler designed his chronograph using the new technology, and it was a huge step forward. It was a neat, innovative design. Doing that with vacuum tubes would have been much more challenging.

So, with new equipment, and less forbidding technical challenges, we started the move away from copper crushers and ballistic pendulums. The equipment and the measurements we have now are much better. For around $5, I can buy a nice instrumentation amplifier chip that has better characteristics than the expensive differential scope plug in that Brownell was using "back when".

That's the link I used, the source of my original post.

I'm not sure when it was sent out, it may be recent.

I wonder if they're trying to reconstitute that technology.

DF

The works of earlier handloaders are, as much as anything, a testimony to the strength of their rifle actions.

Thanks Denton.

And that, gentlemen, is a beautiful thing! Progress isn't always bad. While most folks do not use or own oscilloscopes, everyone can appreciate the advances that have provided affordable chronographs, digital scales and inexpensive borescopes. They may not be lab accurate, but they are perfect for shooters and reloaders to use and enjoy!

The bottom of that page says copyright 2020, so it may be a revisit.

Sorta curious why they would be pushing that. Seems it's not new technology, but a retread of older stuff.

Someone there must think there's merit in this idea, or why would they roll it out in 2020...

DF

I would email them and ask.

Good idea.

Just send them this:


I'm curious why you guys are rolling out the blade micrometer method of checking case pressure.

Wasn't this a big deal a decade or more ago?

Do y'all have new info, breathing new life into this old technology?

Thanks,


Will report their response.

DF

DF - since you now have a fancy blade mike, you really need find out what your Crudmoor prefers: 0.2639" 0.2640 or 0.2641 diameter bullets!

It will be interesting to read their response. 😊

It is my contention that in rifles stronger than the brass, the brass is the weakest link.
Loads should be designed for those rifles that have adequate safety margin so the brass has long life.

a) Looking at Mauser case heads with large Boxer primer pockets we see cartridges like 22-250, 243, 6mm Rem, 250 Savage, 257 Roberts, 25-06, 260 Rem, 6.5x55 [US brass], 270, 7mm-08, 7x57mm, 280, 308, 30-06, 8x57mm, 338F, 358, and 35W.
While the 8x57 may be registered at 35,000 psi and the 270 at 65,000 psi, I have overloaded both until the primer pocket gets loose, and it is at the same predicted pressure in Quickload, of over 70kpsi and less than 80kpsi.

b) The belted magnum case heads can take more pressure than the Mauser case heads, but they are never registered at more than 65,000 psi.

c) The 223 case heads can take more pressure than the belted magnums before they get a loose primer pocket, but are somehow always registered at less than the Mauser and belted case heads.

d) 6mmBR case head and rimmed brass are not the weak link, but rather the primer piercing is. The exceptions include 1) brass flowing into a poor fitting extractor, 2) a 357 mag exceeding elastic deformation of the brass but not the cylinder steel resulting in stuck cases for finger ejection and 3) Norma rimmed rifle brass that is soft.

Working up to lose primer pockets [primer falling out or feeling easy to deprime ] is erratic and not very repeatable. This is because the pockets do not expand uniformly.
The problem with this is that the pocket may expand in places, but the spent primer is still captured by other parts of the pocket.

While it might be possible to check the pocket with a "D" shaped gauge, I have found a convenient predictor is any growth in the extractor groove as measured with dial caliper measuring with 180 degrees of rotation to check 360 degrees. This is reproducible to within a fraction of a grain of powder.


What does it all mean? Answering the engineering question, "What are you trying to do?" If the problem is loose primer pockets, measure the threshold of loose primer pockets with a sensitive instrument. Do not measure pressure.

The problem with CHE is that everyone wants to make it into something that it isn't, & that is a finite, definitive method of measuring pressure, especially the statisticians.....................................

MM

Earlier in the thread I believe a statistician said it wasn't.

Most of the time, when dealing with hand loading, it seems that people claim things to be finite and definite, when they aren't.

Exactly my point..............but he wants the data to show that it is, but it can't, so therefore it has no value. According to his data.

MM

It has been my experience that a lot of handloaders don't understand everything that they're doing or carp what they have read somewhere.

Well, I dunno about that, but surely many don't know all that they understand................is that true in CanuckLand as well or do all north of the border understand all that they know there?

MM

Assuming I can use it well enough to tell....

My Creed likes about everything, so with such subtle group differences, the statistical burden will be a tough one to ascertain a confidence value with a less than massive cohort.

DF

Yep, 3 or even 5 readings don't count much' you gotta me able to measure the same piece 10 times with all measurements less than a tenth difference.

Just sayin'.......................

MM

Forgive me if I'm being obtuse. Why must he want the data to show any particular thing?

Best data is at the target....

Tells me about all I need to know.

Chrono is but icing on the cake.

DF

Think about it for a while from the standpoint of what the technique is useful for & what the data shows that it is not capable of doing, as it does have some value if used with some common sense.

Hodgdon & others are not completely ignorant.

Might come to you.........................

MM

Borders aren't part of the equation. Reloaders are however.

Lots of things change at the border; I don't know whether or not reloading is one of them as I haven't ever had that particular discussion in depth with a Canadian before.

And since we had a plant in Canada, once upon a time, & I spent a lot of time there, I have had some in-depth discussions with my friends there on more than one occasion.

MM

same here

i always appreciate your insights

thank you

Location, location, location

I have always felt, while case head expansion may not tell what the pressure might be, it can definitely tell you when pressure is too high. By the way, I disagree with the idea of using new brass. The case will always expand to some extent, ahead of the web. This expansion will distort the case at the solid head and may give a false reading. Once fired brass, neck-sized, is better, IMO. Generally, any expansion over a couple tenths, is too much. Primers will loosen pretty soon. GD

That (a couple of tenths) does sound like it would be just a bit excessive.

So is zero the bogey ?????

MM

It seems to me that the mistake is in thinking that CHE will correlate with actual measured chamber pressure. Given variances in brass YS, thickness and other aspects of the overall geometry (rimmed vs rimless for example) that will clearly not be the case. However, if you are instead looking for some sort of indicator that your load - regardless of the actual pressure - is pushing the boundaries of safety, then CHE is a valid means of doing that. As someone stated above the case is generally the weak point in a modern rifle, though the primer way also be a weak point.

If you are pushing pressures in a particular case/rifle combination high enough to yield the case head (ie show up as expansion), you are pushing a bit hard. That is regardless of the actual instrument pressure, because pressure is only really a proxy for what is really important here, and that is "will the case fail". If there's appreciable plastic strain (eg primers drop out, hard to extract, case head shows signs of marking from plastic flow into ejector etc) then you really are pushing very hard.

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.