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

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 We dont even have access to an outdated copper crusher set up.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.