Home Forums Hunting & Shooting Ask The Gunwriters Mule Deer and Pressure Testing

Mule Deer:

I seem to remember reading that you have, or had, access to pressure testing equipment at a powder manufacturing facility.

What I am interested in, and someone might have done this in the past, is seeing pictures of the heads of a common cartridge, say .30-06 or .308, that were fired in a pressure testing device.

I would like to see the pictures of the cases, where the first firings would be something like a factory cartridge, then gradually increase the powder charge until pressures became quite high, but not so high as to damage the pressure gun, but maybe, if it would not damage the gun, even to the point where the primer pocket expanded and gas leaked.

I would like to see this to be able to compare the conditions of the case heads as the pressures increased at a steady rate to a known level. Granted, there might not be any difference in the appearance of a 50,000 PSI cartridge case head and a 75,000 PSI cartridge case head, but I would like to see them lined up side by side and a comparison made. Using new cases for each shot.

A lot of pressure guesses are made based on the condition of the fired primer, for example flattened, cratered, normal, blown, and other things. I would like to compare the condition of the fired primers when the pressure was increased and was actually documented. Also, I would like to see notes made on case head expansion as the pressures increased.

Have you ever made any tests along these line? If you have, and have any pictures or information on the test, could you post them?

If feasable, I would also like to see an additional test made, except using a belted cartridge.

I'm willing the bet that the lab was Western Powders in W. Dakota.

I don't know if your wish will come true. Old school pressure guestimation is just that. Just because a case looked like "This" in a pressure test barrel doesn't mean that the same amount of pressure will make the case look the same in YOUR sporter. Common sense and a chronograph is really all the working stiff has thats a somewhat reliable method.

Pretty much what Dan said.

First, no lab is going to risk their very expensive pressure-testing equipment by abusing it very much with with 75,000 psi pressures.

Second, I already kind of did run a test like that. I worked up loads in 3 different cartridges (.22 Hornet, .270 Winchester and .30-06) using all the old-fashioned methods: bolt lift, primer appearance, measuring the case head, etc. Then I had the same "maximum" loads tested at the Western Powders lab. If I remember correctly, in the .22 Hornet the pressures of my handloads were about right, in the .270 they were too high (around 68,000 psi), and in the .30-06 a little low.

The worst thing you can count on is primer appearance. How much primers flatten, or how much the metal "cups" around the firing pin indent, will vary considerably for reasons that don't have anything to do with pressure.

If the head of the case starts showing ejector marks from brass flowing into the ejector hole or slot, then you are running around 70,000 psi already.

The best predicter of pressure is a chronograph. If you're getting muzzle velocities noticeably higher than is common for that cartridge, powder and bullet, then your pressures are higher than they should be. It's that simple. If you work up loads using that rule, then you don't need to look at fired cases, because the pressure will never get high enough to form ejector-hole marks on the case heads or blow primers.

I seem to recall that someone did a similar test many years, and wrote up the results in a magazine. The test was to gradually increase loads, look for "visible signs of pressure", and measure at what actual pressure point these occurred. I remember clearly that one of the guns (a 30-06) did not display "visible signs of pressure" until somewhere around 85,000 psi. Even since reading that article, I've followed Mule Deer's advice: the chronograph.

Bob, that is kind of what I had in mind, if there was very much visable change as pressures increased at a known rate or amount.

The chronograph might be all that most of us have, but the difference between a long throated and short throated chamber can affect both velocity and pressure, and you won't be able to detect the higher pressure with the short throat from chronograph readings. That is one place where I consider the chronograph as being less than an ideal pressure predicter.

Another place is if you are loading IMR 3031 in your .300 Weatherby instead of IMR 4831 or H1000, and you are getting 3200 FPS with a 180 grain bullet, then your pressures are in the safe range because the loading manuals show that you can get that velocity with safe pressures. You just neglected to read the part about using a powder with the proper burning rate for the proper velocity and pressure.

Yeah, you can "see" the higher pressure in a short-throat rifle with a chronograph. The muzzle velocity will be higher.

I had a short-throated custom .270 Weatherby for a while and factory loads were about 150 fps faster than in a standard Weatherby chamber.

John Ricks, Charlie Sisk, and Stan Watson also did pressure tests a few years ago with an Oehler 43 PBL. They all seemed to share MD's conclusions. Usually, by the time you see any conventional pressure signs you are well over 65,000 PSI.

I am able to answer about 80% of my handloading questions by consulting Earl Naramore's "Principles and Practice of Loading Ammunition." This book was first printed in 1954; my second edition was printed in 1962. This book is getting expensive on the used market but your librarian should be able to obtain a copy on an interlibrary loan. The book requires study and you should discuss extending your loan of the book at the time you request it. My local library is very reasonable about this when I ask in advance.

Chapter 34: "Inspection of Fired Cases" discusses case separation and brass flow.
Chapter 47: "The Estimation of Pressures" has a picture on page 824 of five cartridge case heads fired under escalating pressure to demonstrate why visual inspection of primers is not a valid indication of pressure signs.

Page 836 has another picture of primers subjected to high pressures and is very informative.

Naramore discussed cartridge case material and construction in great detail but he does not answer all of our questions about the visual inspection of cartridge case heads.
The Speer No. 14 reloading manual has a picture of three 44 magnum cartridge cases fired at 31,800, 39,000 and 47,700 with little to no visual difference between them.

We need to be careful when judging visual indications of high pressure as I have seen a Winchester Model 70 chambered for the 6.5 X 55 cartridge which had rough firing pin and ejector holes and the bolt would rub a shiny spot on the primer and the case heads when the bolt rotated during extraction.

This is all very cool, but I have a "fast" barell so 10% over max is safe in my gun . . . . .

BMT

I have done a lot of pressure experimenting with Charlie Sisk too, but with his Pressure Trace equipment, not an Oehler.

Yes, once in a while we'll also get a "false" reading from a burr around an ejector hole or slot. I have seen this both with Remington 700's and comercial 98 Mausers. In each case the muzzle velocity wasn't near high enough for such indications, so I examined the the bolt faces and solved the "pressure" problems with a little polishing.

Troublemaker!

This is a Varmint Hunter article on the unreliability of one popular method of estimating pressure from cases.

I have about the same respect for "reading" primers.

GUILTY!

BMT

Actually, I have a bunch of fast-barreled centerfire rifles. But I'm just slow!

My 300 WBY reamer is a short-throat as well. I bought a box of factory WBY ammo (150 grain Hornady loads) for testing and using as benchmarks. Out of my 27" barrel, the 150s were running 3710 at the chrono! I fired two to make sure the first one was a good reading- I think I still 18 factory rounds left in that box. Pressures have got to be high with velocities like that~!

I always specify a fast barrel whenever I buy a rifle or order a barrel............

The above just goes to show that each firearm is its' own law and they are all different. I generaly stick to a loading manual's recommmended charges and if I need a higher velocity, I'll select a larger cartridge.