I recently stuck a strain gauge on the outside of my 30-06, and set off to the range to learn about chamber pressure. I'm using the Fabrique Scientific system, which is quite inexpensive. (www.scientificfabrique.com)

The first thing I checked is the repeatability of the measurement system. Repeatability was about the same as published data for the piezoelectric system, so it's a reasonably precise tool. It's not perfect, but it is adequate.

I did the math to get a ballpark calibration from the physical properties of steel and the dimensions of the barrel. My intent was to fire some loads with known pressures, and verify the calibration from that. I randomized, to swamp out any lurking variables, like barrel temperature.

There isn't any doubt that the strain gauge system is giving good relative measurements. I can reliably tell which rounds produce more pressure than others. Also, the basic physics of the situation indicate a very linear response.

To my complete amazement, the microstrain measurements do not correlate in the least with the published pressures, but the answers were very self consistent. Also, the two batches of RL22 I tested produced very different pressures and muzzle velocities.

So I'm thinking that batch to batch variation in propellants is probably quite large, and that the figures we see in books aren't averages, but rather limits that we won't exceed if we follow the recipes posted, based on known variation in propellants. If that is true, then:

1. You could usually exceed the posted charge without causing a problem, because it's a pressure that you'd almost never achieve, but might reach if you got just the right jar of powder (I'm not advocating this!!).

2. Different technicians would find different maximum charges for the same bullet and powder, and published maxima would vary from book to book.

3. The published muzzle velocities would practically always be higher than what reloaders achieved in practice.

4. Some people would swear by a given recipe, while others would claim that it wasn't worth a dang, because of differences in the raw material they use.

All of that matches reality.

Anybody got some helpful information they can contribute??

Only a couple questions because this pressure thing has interested me for years when trying to determine max. psi for a load. I know it would be diffrent in my -06 so what did you find with loads you tested and what were the pressures and what load bullet, powder etc..
thanks Dave

150 Hornady SP, 3.210" COL, 63 grains RL22, Fed 210: Jar A, 46,502 PSI, 2884 fps. Jar B, 43,098 PSI, 2805 fps.

150 Hornady SP, 3.210" COL, 63.5 grains RL19, Fed 210: 53,100 PSI, 2957 fps.

165 Speer SP, 3.250" COL, 62 grains RL22, Fed 210: Jar A, 49,651 PSI, 2832 fps. Jar B, 47,673 PSI, 2781 fps.

180 Speer SP, 3.250" COL, 48.5 grains RL15, Fed 210: 47,968 PSI, 2576 fps.

180 Speer SP, 3.250" COL, 60 grsins RL19, CCI200: 55,703 PSI, 2765 PSI.

MV standard deviations range from 4.8 to 10.8, except the last load, which is 25.8.

Warnings: Pressures are relative pressures only. Absolute calibration has not been confirmed. You can believe differences between the pressures loads produce, but the whole scale can slide up or down. Some subgroups are very small.

If my thinking turns out to be correct, it may be much more the difference between which lots of powder we got, than the difference between the cut of our chambers.

Denton, can you explain how a strain gauge works? Also, can you explain how the piezoelectric works?

Thanks,
Casey

A strain gauge is a resistance based device, which you epoxy to your barrel, about midway in the chamber. When the gun fires, the OD of your chamber grows about .0005" in diameter from the pressure. The resistance of the strain gauge changes as it and the barrel are stretched. Basically, you make the gauge part of an array of fixed resistors, and apply a fixed voltage. As the barrel stretches, the voltage across the strain gauge changes by a few millivolts. You amplify those millivolts, and electronically capture the peak. This number is directly proportional to peak chamber pressure.

The piezoelectric device is based on the same principle as older model microphones and phonograph cartridges. Certain crystal materials generate a voltage when they are deformed. You drill a little hole in the barrel, and use the piezoelectric device to generate a voltage that is proportional to pressure. You amplify that voltage, which is proportional to pressure.

It sounds complex, but the circuitry is really pretty simple. Modern integrated circuits make it much easier than it was a few decades ago.

The strain gauge system is one that a home hobbyist can use, without messing up a gun. I have two rifles that I have equipped with strain gauges, out of sight, under the stock. I have a small hole drilled in the stock, and when I want to take measurements, I just plug the strain gauge meter into a socket epoxied in the hole.

Now, when I get a new jar of powder, I can work up a load very quickly. It's also far more accurate than looking for pressure signs. Your pressure sign is a high reading on the meter, and you can track actual pressure as you increase powder.

Getting a chronograph is a real eye opener. That's "eye", singular. Getting a strain gauge opens the other eye. It is quite revealing.

I've almost given up on Aliant and their Reloader series of powders...they are consisently the Most Inconsistent powders I've ever used...every time you change lot numbers you need to begin all over. I've had one lot of RL15 show big-time pressure signs at 2450fps in my .375 with the 300gr Nosler while another lot allowed to go almost to 2600 fps with no pressure signs..and the amounts of powder were within 1 grain of each other.

DB Bill--

Interesting comment. I checked, and I have three different lots of RL22 on hand, and three different lots of H4350. Wonder if I should whip a quick F test on them, to see which has the nost lot to lot variation.... Hodgdon claims that they are very consistent. Three batches isn't enough to make a solid case, but it might be interesting....

As reloaders get better instrumentation, we can do a better job of evaluating suppliers. Where appropriate, we can put marketplace pressure on them to improve their products.

I don't know if you followed it last year or not but one lot of RL22 was pulled from the shelves and if had powder from this lot and called Aliant they told you to get rid of it and either sent you replacement powder or a voucher for powder at your local store. They refused to call it a recall but if it walks like a duck and quacks like a duck I think it's a duck! The problem came to light when a 1000-yard shooter froze his bolt with a load that had, with previous lots of RL22, been 10% under his competition load (not what he thought was max but his most consistent load). Aliant tried to sluff it off as a fluke but enough others came to light to trigger their
"it's not a recall" reaction.