Home Forums Hunting & Shooting Ask The Gunwriters Question about velocity/pressure

Using pressure testing equipment (in my case an Oehler 43) can be a very humbling experience for a handloader. The number of variables can almost seem infinite. The number of different types of chambers, barrels, powder, primers, bullets, cases and the means they can be assembled is just a start. And then add in lot to lot variations and the fun compounds.

Logger
You got that right !

Last time I saw John Linebaugh we talked and looked at numbers from Western Powders.

I don’t show them to folks much. Just eye openers.

And that’s in a 500L……..same bullet weight and velocity……..except one was 35k psi…..one was 80k

John loved the study of pressure and velocity. Moving out there and looking forward to reading in his library.

Precision Shooting is another pleasant read.

Thank you for this thread.

One of the tests I performed at the Wester Powders piezo lab dealt with the "accuracy" of the long-time pressure-signs still used by many handloaders. I handloaded some rounds for three different rifle cartridges using such signs as flattened primers, case-head expansion, sticky extraction, and ejector-hole marks on the case heads.

Then I loaded up new cases of the same brand with all three loads and we tested 'em in the Miles City lab. It turned out that one load just about spot-on, another was way under SAAMI pressure, and the third was way over SAAMI pressure.

Ran some other tests in their lab back then, and it turned out that most "pressure signs" except primer flatness don't show up until around 70,000 PSI average maximum pressure--which is 5000 PSI more than SAAMI allows for any cartridge. Primer flatness, however, can occur even with pressures in the 50,000 PSI range, usually due to a little more headspace than SAAMI specs allow.

Any of this pressure peak variance have anything to do with barrel harmonics/vibration? JB, any notice of a more consistent pressure curve with a full-length bedded NULA vs. something with a generous free-float or tip-pressure?

I have tested that and found no correlation .

And there's no reason there should be any....

Thanks to CharlieSisk and the others for their contributions! I don't have much to contribute, but have a couple other questions:

Would this be a factor in the 7mm Rem Mag's lower SAAMI MAP pressure? I've read how wilder pressure swings in that round have been cited as a reason for 62K psi vs 65.

What sort of pressure testing equipment would your recommend these days to someone who wanted to dabble in testing?
Also, when you mention the electric grid swings, are you saying the variables in grid AC power introduced inconsistencies into your results?
This is good stuff, thank you for posting.

Interesting thread. Can you (John or Charlie) tell if you have experienced a pressure change with faster or slower twist rates?
It was an argument in another thread here years ago.

Jeez - it's a wonder the average joe reloader can get things to work as well as they do.

PGoat
I have not experienced these swings in 7mm RemMag. But...(dam thing came up again )...the testing I have done in this caliber is with the rifles I have built. Properly cut chambers, good barrels, etc. With good components it seems to work as well as any other.
Except for one time....I had a batch of new Norma or Lapua brass in 7mm Rem Mag. Built a rifle, it shot like $h!#!. Velocity, accuracy, pressure, all over the place. Then, I learned about annealing, and all those problems went away.

I have not experienced any connection between twist rates. That doesn't mean its not there, but I cant find it.

Twist has a very minimal to no effect on pressure

Most electronic test equipment has stabilized distribution transformers, and in most cases are DC. NMR,IR,Mass Spect., Xray diffraction. These usually cost more than the measuring device. It depends on how many zeros to the right you want to go. The pressure equipment uses an algorithm that is built into the chip, and gives the time/length pressure curve seen on the observation device. If measurement interval is made smaller this curve will have peaks in it not "smoothed" out by the fitting log formula.

Understatement--- good thing those random errors cancel out the systematic ones.

I have a strain pressure measuring system. It's a Faber unit before Ristow offered his Pressure Trace (I also have an unused PT). The Faber captures Peak Pressures (actually it produces a number that he termed "microstrain" that I have to convert to psi via a closed vessel formula. I primarily used it to work up service rifle loads for the 90gr JLK (to keep from blowing ourselves up in uncharted territory).

Funny I just decided I needed to get a PTII and found they're discontinued. I have an email in with Schaefer with fingers crossed.

I'm not sure that is a question that strain measurement can answer definitively. Mounting a gage on a barrel makes it a unique system that provides strain measurements. You then have to calculate the pressure given a multitude of factors such as barrel dimension, brass hardness etc etc. Mount a gage on a different barrel and you've created a different system subject to it's own calibration etc. Calibration between barrels (systems) is a challenge unless you're prepared to use a high pressure oil system as Harold Vaughn did or have access to expensive high pressure measurement equipment. I had a reference lot of ammunition that I'd fire before each session to ensure all was as expected. As imperfect as that was, it was as simple and practical as I was willing to go. Ristow felt that his computer algorithm did a good enough job with calibration. I was skeptical.

From a purely engineering perspective, the energy required to increase spin on a bullet (for us it was going from 1/8 or 1/7 to 1/6.5 was neglible. This was borne out in our firing tests. Same load in same chamber design shot to pretty much the same velocity whether it was in an 8, 7 or 6.5 twist.

Mr. Sisk, I'm curious how you're calibrating your PT? Are you going with the software calibration?

Most experienced handloaders eventually learn that there are standardized or common velocity thresholds where pressure safety and accuracy can be expected as a reasonable outcome. longer case life will support those velocity ranges per cartridge and the loading manuals have been teaching us that for decades if we listen.

Once a hunter learns that chasing that extra 100fps doesn't mean much in the field and could be the reason that the extra level of accuracy you are wanting is absent, things get a lot easier and a lot safer for the rifleman and his components.

If you want magnum velocity, great, but get it from a magnum case. Life can be simple if you're smarter about it.

Well and succinctly said, AGW.

Thanks Charlie and Chris. My thoughts were same, jwp. The amount of energy to twist it IS FAR less than needed to launch it. But inanet warriors thought otherwise.
And, Chris, yes it would be very hard to test for being in different barrels.
Thanks all.