Home Forums Hunting & Shooting Ask The Gunwriters Practical pressure testing for handloaders

Is there any for sale pressure testing equipment or testing labs that can determine pressure from my handloads in my rifle that are available to a common handloader who cannot spend a fortune on testing costs or equipment? Rifle is a New Haven Win Model 70 that formerly was a 30-06. It now wears a long throated former 7mm-08 Win 70 factory 22" barrel chambered in 7x57AI. Chamber was cut to allow 175 grain bullets to be seated to full magazine length for the original 30-06 length magazine. Load development so far indicates that only the 120 grain bullets I have tried do not end up inside one inch groups at 100 yards. 140 to 175 grain bullets all seem very accurate and groups seem to shrink as powder charges increase. So far I am not experiencing any visual case signs of too much pressure such as markings on cases or heavy bolt lift and I have up to four loadings on Win cases with primer pockets still tight. Trying to find max hunting accuracy loads with a variety of bullets without having any problems or accidents.

Pressure Trace II works extremely well. Costs roughly what a decent, not entry-level, rifle costs.

Recreational Software, Inc (RSI) is where you get them from.

rflshtr,

You didn't say a word about your chronograph results.

If your velocities are reasonable and as expected, you can be pretty assured that your pressures are, too.

If you are getting expected, reasonable velocities with good case life you don't need to worry about your pressures, in my opinion.

I am pretty much in agreement with your statement, PROVIDED the handloader is working with published load data, and is using the same components as the manual. If you are substituting components, though, stuff can get bad real fast.

I had a fellow come in to my ER some years back when the 7mm STW was a new and hot ticket. He was using published data, but he didn't have the same bullet as the load manual used, and he didn't have the same primers. So he was basically wildcatting without acknowledging the fact. His rifle barrel split into 3 sections, shattering the forend and shredding his left hand as badly as you'd never want to happen to you. He did not admit at the time that he had "colored outside the lines" until months later, when I ran into him at the range one day. He had been chasing book max velocity using his chronograph, which was IMHO a safe way to go, provided you haven't substituted components. But the bullet he was using was a very different bullet than the recipe, with a much longer bearing surface. That and the hotter primer he used presumably blew up his rifle. The hand surgeon who reassembled his hand spent almost 4 hours in the OR fixing the mess, and there was a long and painful rehab process. Not a road I ever want to see a friend go down again.

Use a chrono, follow the manual religiously, and you can judiciously play around the edges of the performance envelope safely and not have to worry about pressures. But if you're going to truly go outside the book, you are smart to pay for piezo electric pressure equipment.

There is one other caveat when using a chronograph to keep pressure in line: Sometimes MV does not advance smoothly with increased charge. If you get into that region, all bets are off.

Pulished load data for a baseline, should be able to be exceded safely due to longer freebore.

Thanks for the replies so far. I cannot find much newer published load data for the 7x57AI and regular 7x57 varies quite a bit. Long throat is a big issue and I want to take advantage of it within safe zones. With my other AI cases, I have found some erratic pressure/powder relationships as powder charge is increased but none of my other chamberings have intentionally long throats like this rifle has. Do not want to get to situation described by DocRocket above but am looking for a safe way to achieve max safe load capability in my rifle without endless trial and error. Most of my wildcats are varmint calibers but I do have a 358 STA that has behaved erratically with data provided by Western Powders and which I originally probably loaded too hot using loads from Layne Simpsons Shooting Times published article which are markedly higher than loads in the most recent major load manuals. I also have some experience with Todd Kindlers loads for 17 and 20 caliber wildcats which seem hot to me in my rifles and some experience with the PO Ackley published loads from his two volume set which are quite old.

Where can a handloader get access to piezo electric pressure testing labs or equipment and what does it cost?

DocRocket, I wonder whether your friend experienced secondary explosion effect. That's what it sounds like. Bad deal, big cases + slow powders.

rlshtr,
Your best option is a Strain based pressure setup. Of the strain based units, Pressure Trace probably the most accessible and supported unit. Piezo is beyond the scope of hobbyists and will require modifying your rifle more than you'd like. If you're just looking for "relative pressure changes" (documenting how pressure changes as you modify your load), strain will do this very well. The challenge of strain (and actually all pressure measurement techniques) is calibration. Without calibration its difficult to put a PSI number on your readings. The RSI software will give you psi, but it is a derived estimate from an uncalibrated setup. Use your chrono data in tandem and some good judgement.

I use VV-165 and 175 grain NP's in my CZ 550 7x57. In the 24" (almost) barrel it's getting 2600 fps average and good accuracy.

This is a 160 grain load,but it shows how long the bullet can be seated out.

There are a couple of paths that you can use.

One is QuickLoad software. If you measure your case capacity and COL, it does a better than fair job of estimating pressure and MV. Over a limited range, you can use the chronograph to fine tune the charge weight to get the same MV as QL estimates. It's a good bet that if your MV is right, your pressure is right.

Note that you will have to account for temperature, since SAAMI does their testing in a temperature controlled environment, and specifies that ammunition must be between 60-80 F.

Your other alternative is the PressureTrace that has been mentioned.

The PressureTrace instructions do indeed say that you should use it only for relative measurements, but that is just legal boilerplate. I knew Burt Mitchell, who developed the PT and even made a couple of suggestions that improved the design. Long story short, if you do things right, the PT is capable of giving well calibrated results. The reported pressure is found by plugging dimensions, the properties of steel, and the amount of strain at the surface of the barrel into the Hoop Strain Equation. The measurement of strain depends on the properties of the strain gauge and the supply voltage. All of the inputs are easily obtained to three significant digits. If the inputs are right, the output is right and the result is accurate (calibrated). Anyway, it isn't the PSI that matters. What matters is the strain that is induced in the steel.

Like the QL approach, you will need to control temperature. The temperatures that matter are the barrel, just forward of the receiver, and the ammunition. Barrel temperature is more important than ammunition temperature.

What you can't do is instrument your rifle with piezoelectric equipment. Piezo equipment involves drilling a hole in your barrel at the chamber.

SAAMI does acknowledge commercially produced ammunition as a secondary standard. That probably isn't going to help you, since your cartridge is AI.

Care to elaborate ?

What powder burn rate do you recommend for big cases ?

Jerry

I should perhaps have said that sometimes that combo is less predictable than expected, and that is what can be a bad deal.

The general concept of secondary explosion effect has, I'm almost certain, been posted about here at various times. I'm almost certain I've seen Mule Deer mention the pressure excursions that happen more frequently (in pressure testing labs) with the 7RM and 243 Win than with the sort of 'average' case.

I expect that my best elaboration would be to refer the reader to a series of articles published in The Handloader magazine from 1967 - 1970. It began with a piece from Colonel G.O. Ashley who was claiming that essentially there was no such thing as S.E.E. Dr. Brownell wrote a reply and began a series of thorough rifle pressure measurements and investigations. These were all available from Wolfe Publishing on a DVD called Firearms Pressure Factors. One can still see it listed on their site, at https://www.riflemagazine.com/catalog/detail.cfm?ProductID=821 However, when I clicked on the more info link earlier today, it went nowhere, so I'm not sure whether it's available any longer or not.

Basically, there are conditions under which pressures can spike. The best I understand it, and there are almost certainly others here who understand it better, the effect seems to be most typically associated with large cases using reduced charges of slow powder. That said, I've recorded (with my PressureTrace) what appeared to me to be the onset of the phenomenon in a very different combination of case, powder quickness, and bullet.

S.E.E. can blow a rifle barrel up, which is something that will not normally happen unless there's a barrel obstruction, or some seriously compromised or defective barrel steel, or the like.

So really, I would simply reinforce the assertion to follow load manual combinations exactly, particularly when using large cases like the one mentioned in DocRocket's post.

I will also mention here that I have sometimes experienced deviations in QuickLOAD such that, even if I've measured and entered everything as carefully as I can, and even if speeds are matching up perfectly, QL's pressure predictions are just _way_ off. I don't say that's the _normal_ thing, but it's happened enough that I will always recommend against using QL for pressure prediction use.

I am aware of "pressure excursions", unexpected jumps in pressure with 'some' unspecified powders. Not all of those occur when increasing the powder charge. Some have occurred when 'trying' reduced powder charges using 'slow burning' powders.

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" pressure excursions that happen more frequently (in pressure testing labs) with the 7RM and 243 Win than with the sort of 'average' case."

Second ? -- what is UN average about the 243 (308) Win case ?


Jerry

Mr Bramwell,
We've had some good discussions, and I respect your knowledge and work, but I disagree with you on two points. QL will give you simulated pressure readings based on their computer modelling and it may work for the majority of situations, but I have run the simulations and the QL algorithm does not account for all the situations. I have run into cases where QL spit out a pressure that was different than strain measured pressures to a degree that was significant from a safety standpoint.

I use strain, and I have a RSI unit as well as one from Steve Faber. No matter how much Ristow swears his algorithms accurately calibrate his strain readings to yield PSI, I've manually calculated the open vessel and LAME pressures and they're only as good as the input. Ristow himself says that the algorithm only works for cylindrical vessels. What do you do for a tapered cartridge or a tapered barrel? I like the resolution and pressure curves that Pressure Trace gives, but I also understand that these pressures are derived and uncalibrated and I'm not inclined to use pressurized oil to calibrate my rigs. With all of that said, PT gives you usable results. I just have a problem trusting running pressures to the upper end without "really" knowing where the upper limit is with the PT (or the Faber). Like I said earlier...use in tandem with your chrono and use common sense.

This is a tangent to the original question.
A while back, there was a guy who went by the handle OK Shooter (his real name escapes me right now). He did some pretty cool work with an Oehler PBL (another strain based pressure measuring system). He took a 30-06 and varied things such as powder type, powder charge, primer brands and type (standard and magnum) as well as seating depth. He measured pressure via strain for each change. It was a pretty cool view of what changes in our "recipes" will get us in terms of pressures. He rolled all his research into a book self published book (more like a binder).

Re: Pressure excusions/spikes ? for MD & others 01/15/10

Myself, I discarded a .243 and choose a 6mm Remington inherited from my wife after stocking up on brass - when she was alive I had a .243 and she had a 6mm Remington in otherwise identical actions. Hers was stocked to fit her so different cartridges wasn't such a bad idea as otherwise confusion might have been an issue. Restocked to fit me with her stock saved for youngsters. The .243 did show odd behavior compared to the 6mm. Again for myself I jump from the .280AI to a 7mm STW ignoring the 7mm Remington Magnum. Explanations have I none.

Given a wildcat rifle like the OP I would settle for loads I felt confident in and ignore pushing pressure limits. If I thought I needed more bullet speed I'd go .280AI or 7X61 Sharpe and Hart which I consider nearly platonic ideals of 7mm bore size.

Well, the sometimes squirrelly pressures are one thing. :-) I'm not positive what the 243's issue is. A little throat wear seems to increase the likelihood of odd pressures. It's quite overbore, of course, but that alone is not a unique characteristic.

I've seen very bad-looking secondary 'pressure spikes' coming on when testing in a very not-overbore cartridge. I didn't care to see whether what the PT was showing me was real, nor if I could increase things to the point of destroying the barrel. I just quit the whole test sequence.

I'll mention something I've noticed for years and I'm sure many others have as well. I'm a conservative handloader and usually load no more than book maximum charges, if that. I've noticed many times when approaching a max. or near max. powder charge, chronograph figures show a big jump, say 80-100 fps by adding one additional grain of powder (I seldom do half grains with any rifle cartridge). Usually, I'll back off one grain to where velocity gain is about 50- 60 fps per grain of powder and call this max. for my rifle. This often coincides roughly with book figures.

Is the big jump in velocity what some here are referring to as a pressure spike?

ChrisF....

Absolutely nothing wrong with a respectful disagreement! Working through those can be very illuminating.

There are many reasons that QL and the PressureTrace might disagree. One is failure to carefully control barrel temperature. Another is individual case capacity, and still others are seating depth, engraving force, and leade. With some common powders, a 30 F change in barrel temperature is equivalent to a grain of powder.

All of these will influence both MV and peak pressure. In my experience (which does not include all possible cartridges and loads) MV will closely track peak pressure, unless you've reached a point that more powder produces little increase in MV. Hence my comment that if your MV is right in QL, it's a good bet that your pressure is right, too.

Finding and reporting exceptions to that is a worthy work.



Easy peasy lemon squeezy. The taper is both small and linear, and the strain gauge is small. Get the ID and OD under the middle of the gauge, and the error on both sides of center will balance out.

If you've got one of the original Faber units, you've been at this a while. Mine is still knocking around in my shop, somewhere.

The so-called "secondary spike" is a real event, but not real pressure. It is entirely different from the explosion phenomenon, which is real pressure. I think it probably originates from the topology of the instrumentation amplifier used in the PressureTrace. It's easy enough to check with a generator, but I haven't done it. I did build up my own high gain diff amp, and put a full strain bridge on the barrel. I could not get any trace of a secondary spike in a rifle that had easily generated them in the past, shooting on several days, with different conditions.

The Secondary Explosion Effect is dangerous. It usually crops up with slow burning powders and less than full cases. In the Brownell work ChrisF cited, you can see that pressure becomes much more erratic below about 30 KPSI. One of his students blew up a rifle by putting a case of powder in the chamber to dislodge a bullet part way down the barrel.

There is yet another effect that should not be ignored. That relates to "ringing" barrels.

Sometimes, you'll find a ring inside the barrel, near the muzzle. If you remove the barrel and set it back a thread, you'll get a new ring the same distance from the chamber. In some cases, it will blow the tip of the barrel off. I know Charlie Sisk has done that about three times.

This probably arises in the same way that the "shock collar" happens on an aircraft or rocket. There is typically about 2 grains of air in a 30 caliber barrel. As the bullet accelerates, that air is inertially compressed. As acceleration decreases and the mass of the remaining air is small, the remaining air expands, which cools it, which drops the nice compressible water vapor into incompressible liquid water. Crunch!