Home Forums Hunting & Shooting Ask The Gunwriters CHE as pressure indicator

I don't wish to engage in a debate over what methods are best for determining "safe" pressures in reloads but would like to ask a question of those who use this particular method. What is the general rule of thumb as to what is acceptable (if any) expansion when measuring the solid part of the case just ahead of the extraction groove? I am assuming virgin brass and measurments occuring at the same place on the case. Again, I realize there was a somewhat heated debate on pressure measurment recently and I am not interested in revisiting that.

This is not a safe or reliable way to keep tabs on pressures. By the time the case head swells enough for the swelling to be measurable by ordinary means and typical handloaders, the pressures are already 'way up in the danger zone.

Sorry � take it or leave it. I'm not going to debate the issue further. I've had to convince somebody or other of this fact, one after another, for many, many years, and I'm worn-out with it. The idea just keeps flaring up like persistent spot fires in a forest fire-fighter's nightmare and always finds a sucker who'll "buy it."

I have experimented with this, by measuring case-heads and then firing rounds from the same batch in a piezo pressure barrel.

Measuring case-heads works OK just often enough to get you into trouble. There is no way to EVER be sure your measurements are keeping you below the red line just by using a micrometer.

MD

This post is going to get me in trouble,but here goes. Ken Waters used CHE as an indicator of excessive pressures throughout his two volume Pet Loads.I've long considered Mr. Waters to be on the conservative side in his loadings in comparison to some members of the gun writing fraternity.

Differing opinions are some of the things that make handloading so interesting.

Ken usually compares the case-head expansion of fired FACTORY rounds and compares this to his handloads--generally by measuring the "expansion ring" in front of the case head, not the case head itself.

You can get somewhere in the ballpark here, but only if the brass is the same. I tested this too and it works OK, but tends toward conservative loads, just as the factories do. Which is probably a smart thing on Ken's part.

But there is no "magic number" in case-head expansion that indicates a safe load.

MD

Here is some science on the matter:

http://www.shootingsoftware.com/ftp/dbramwell%20july%2019%2004.pdf

Jackfish: Thank you for that reference to a data-based article. Even with my dim recall of the past raucous debate about the merits of CHE and PRE vs. the pressure trace system, this one article best summarizes the whole argument. I forget who said it, but "Facts are stubborn things." I've had to reclassify some of my cherished references to "historical interest only" in the light of better information. Speaking only for my pain-phobic self, I'd rather change my mind because I had read an article than because I had hurt myself and/or somebody else by following incorrect information.

This question was posed for those who use CHE as a measurement for determining max pressure or to detect when a particular load was getting too warm. I realize that those of you who have answered consider it a "fools" technique. So be it. I don't have a position one way or the other. What I do have is a rifle (280 Rem) that does not seem to answer to the reloading manuals max load velocities. I have reloaded long enough to recognize the presence of many variables when assembling loads and even when allowing for these velocities are far below spec. I am not trying to push the envelope with this rifle but simply trying to realize the natural potential. So, I am left with working up loads that surpass what is listed in manuals or trash the barrel and start over again. I am watching for any of the more traditional signs of pressure (I have had enough of these over the years to recognize them) which have not showed up. Short of buying pressure testing equipment I have little recourse. By the way, it does seem intuitive to me that one might use 270 reloading data when using bullets of equal weight and construction ie. 150 Nosler partition bullets and I do notice that most manuals do not reflect this when dealing with the 280. I do realize that the round is downloaded for operation in pumps and semi autos. No other rifle I have ever owned manifested this behavior. Any theories? The chronograph is an Oehler and has given consistent measurements over the years ( reasonably consistent with expectations).

msc:
You might try an e-mail or telephone call to Hodgdon Powder Co. Hodgdon's web site has the appropriate e-mail address and telephone number. Those folks have a good reputation for responding to consumer questions about the safe use of their products. Hodgdon used to test loads for individual consumers, for a fee. I don't know whether Hodgdon still offers that service.

msc, I don`t think 280 and 270 data will interchange well. There is a differance in expansion ratio due to bore diameter differance. They are twoo different cartridges.
How does your rifle do with factory ammo as far as velocity? Does your handloads velocity compare with the factory or is it as you say "far below spec"? Factory saami pressure spec is 60,000psi so I would expect it to be at or near the max velocity you`ll get with top book loads useing the better powder choices.

I have one of those "slow barrel" guns and I know why it is so. The gun has a chamber that is somewhat liberal in chamber dimension(determined with a Cerro-Safe cast), and conservative in bore dimension, being .002" smaller than standard bore for caliber. I got nervous before ambition took charge and began to investigate the WHY rather than try to match book velocities.

Because of the dimensional variations it is easy to load max. quantites in the once fired cases, even in this capacity challenged case. The chronographed velocities run 100-200 below what I expected based on the load manuals. PRIOR to reaching Max. loads from any manual I began to see impressions on the base of case heads that indicated flow into the ejector cutout. Bolt lift was fine, extraction a slick little finger tug on the bolt. There was NO measurable increase on CHE/PRE between once and twice fired brass. Or first and 5th fired brass.

From that experience I have determined that PRE/CHE is of little value as compared with my face. I have also determined that I will not ignore any of the conventional warning signs either, even as I work up loads that are milder than manual max. When things are going faster, or slower, there is a physical reason. It ain't voodoo....... <img src="/ubbthreads/images/graemlins/smirk.gif" alt="" />

Oh, the gun is a Ruger 77 in .358 Win. I would not sell it or change it in any way, as it is very accurate, and hits like one of Thor's minor hammers.

msc--

What velocities are you getting in the .280? A chronograph is a fairly accurate measure of pressure, in my experience probably the best one the home handloader can use.

MD

For completeness sake, if you buy a copy of Quickload, and calibrate with a good chronograph, you will be able to determine your operating pressure to within a couple of percent of actual.

Or, if you can find it, use 7mmx64 CIP data: Same cartridge, essentially, higher max average pressure..... FWIW, Dutch.

One point is worth mentioning; When the case head begins to
expaand you are reaching the yeild point in the brass. That is
a good time to reflect on your mortality. Who cares what the
precise pressure is?.

Good luck!

Dutch--

When you say "calibrate" QL with a good chronograph, just what do you mean? Adjust the load on QL until it matches the actual muzzle velocity in your rifle?

MD

Am still waiting to hear from Dutch on the use of QL and a chronograph to calculate pressure within 2%.

This would be a major miracle, for two reasons. One, an experiment done by A-Square a few years ago found that merely changing the primer in a .300 Winchester Magnum load changed pressures up to nearly 13,000 psi--while velocity only changed less than 50 fps. QuickLoad doesn't even consider the primer in its calculations.

Two: Such a breakthrough would render all pressure-testing equipment immediately obsolete. That doesn't appear to have happened. In fact all the pressure labs I'm in contact with report QuickLoad as a VERY approximate program.

MD

well, 'scuze me for having to get some work done.....

What I intended so say is that if you calibrate Quickload to a chronograph (i.e. adjust the settings within the program so that observed velocities match predicted velocities), any subsequent predictions of velocity, in my experience, are within 2% of observed velocities, even when changing components. To me, that is pretty astounding. (I'm well aware of Quickloads problems with straight walled cases -- but the previous statement has held, for me, on "normal" shaped cartridges).

Forgive me for being sceptical about anything AA publishes, but the point is valid. There are plenty of examples illustrating the changes in pressure due to changes in primers (Ramshot's reloading guide #II has one with the 223) or other components.

And, while the point is interesting and suggestive, no one has bothered to compare those observations with Quickload predictions. In the case mentioned, did Quickload predict the High Pressure, the Low, or somewhere in between? If Quickload predicted the low, I'd be concerned, if it predicted the average, not so much, and if it predicted the high, well, Kumbaya.

Let's not over-complicate things. The essence (sp) of a primer change is a resulting change in the shape of the pressure curve. If you boil it down to the basics, changing the primer changes the burn rate of a powder. Again, I am not worried about false high predictions, I am worried about false LOW predictions (VERY likely when measuring cases, based in some pretty good investigations by OKshooter, and Denton -- and your own published work).

Quickload has plenty of room for improvement, but in combination with some critical thought and a chronograph (and short of a strain gauge or piezo), it is an order of magnitude more accurate than anyone with a micrometer. I'm sure it has quirks I have not yet run into. I'm sure I'm going to be off 10% or maybe even 15% one of these days. That's 5,000 to 8,000 PSI or so: I don't play that close to the edge, so really don't care.

I'd love to see someone run Quickload up against a strain gauge or piezo. I would be rather surprised if Quickload predictions would get someone in trouble.

As a matter of fact, I'd a lot rather read that write-up (even, make that PARTICULARLY, if it shows I'm wrong) than another write-up that Nosler's new rifle actually does go "bang" when you pull the trigger, or that the latest Hornady bullet does put a hole in an animal when you point it straight. JMO, Dutch.

The original question was with regards to the acceptable amount of expansion. I'm assuming you are referring to measurement of the solid head of the case and the answer is... None!
If the case head expands by .0005" the first time, it is likely to expand by as much the second time. One more loading and the primer will fall out. This is obviously too hot. Brass should fail from work hardening or wear not because the primer pocket expands.
So measurement of the solid head of the case lets you know when you have gone too far.
Up to a certain point, measurement of the so-called pressure ring may provide a basis for comparison but I never thought much of it and still don't.
Rocky Gibbs' criteria of "good case life with repeated reloading" is probably as valid as any other concepts.GD

This is a quote from Ken Howell in the Big Game reloading thread "pressure vs. Velocity" regarding calibrating QL.

"In the lower right corner of the upper left QuickLOAD window, look for the "weighting factor." Adjust (vary) that number until QuickLOAD predicts the same or nearly the same velocity that your chronograph reports.

Remember, BTW, that QuickLOAD predicts muzzle velocity, and the chronograph reports a somewhat lower instrumental velocity.

Once you've thus "tweaked" QuickLOAD to best fit it to one load in your cartridge in your rifle, the software should be a reasonably good guide to what velocities and pressures you can expect with other loads in the same cartridge and rifle."

Just thought I'd put these two statements together as they are talking about the same thing.

Dutch--

I share your skepticism concerning AA, but have seen too many similar results elsewhere (including my time spent in pressure labs) to doubt his results.

I recently ran some QL predictions, set up the way you describe, against some results from both peizo and strain-gauge data. Both pressure machines had been corrected with reference ammo:

.30-06/165 Nosler Partition/58.5 IMR4350--2884 fps
Piezo 58,348
QL 56,616

.22 Hornet/ 40 Ballistic Tip/ 11.5 Ramshot Enforcer--2951 fps
Peizo 47,770
QL 58,846

.270 Win./130 Hornady/ 58.5 RL19--3169 fps
Piezo 67,291
QL 62,291

9.3x62/250 Barnes X/ 64-RL15--2600 fps
Strain 47,260
QL 59,033

.300 Win. Mag./180 Nosler Part./75-H4831SC--2969 fps
Strain 63,800
QL 57,264

The closest results were with the .30-06 load, about a 3% difference. The others ranged from 8% to over 25%. Please note that QL underestimated in three cases, the error ranging from 3% to over 11%.

Yhe vast differences in the .22 Hornet and 9.3x62 results are probably due to QuickLoad's problems with straight cases. Neither has much shoulder. Probably we shouldn't even rely on QL too much with that type of case.

Just might write it up. Perhaps you missed my column in HANDLOADER explaining the difficulties of using a computer program to accurately predict pressures, but this might help grasp the principle.

MD