Pressure Measurements - how do you do it...?

[AJMD429]

Sometimes I'll see folks post about how such-and-such a load produced "1,200 fps at 24,000 psi".....

I have a chronograph, and can measure the velocity, but as I understand it (...which is about the same level a snail understands world geography... ) there are two main ways to measure 'pressure':

> a special made barrel with a pit for a copper pellet that will crush a predictable amount per unit of pressure, or
> a barrel 'strain-gauge' that is applied to an existing barrel at the chamber area.

So I'm assuming that most of the figures quoted are from the second method, or another I am unaware of, since using special pressure-testing barrels would be expensive and not very practical.

Or....maybe they are quoting QuickLoad predicted-pressures....?

So that leads to my SECOND question:

How do those strain-gauges really work...it looks like you apply a little plastic and metal-strip thingy to the chamber area, and the 'stretch' that happens when you shoot will give a 'PSI' reading.

Seriously....?????? I know technology is really cool and stuff, but I would think the amount of stretch at firing of a rifle barrel at the chamber would be super-small, and any change in circumference small as well, so the applied strip thingy would have to be able to measure millionths of an inch of stretch, and discern say seven millionths from eight millionths, just to give one-digit PSI accuracy. To differentiate 44,000 from 45,000 PSI, the strip would have to be able to discern stretch distances within a minimum of three digit accuracy. Is that really possible...? Wouldn't the elasticity of the adhesive muddy the water quite a bit....?

Anyone who can expound on this with something sophisticated enough to be believable, but simple enough for a dummy like me to understand it, would be worth reading the posts of...!

[.45colt]

Doc, look up Savvy Jack, He is a 44-40 lunatic , and He knows how this All works and has a pressure test barrel. He has posted Here quite a few times lately. He is a Great Hombre. .

[JimT]

The amount of movement may be more than most folks realize. Years ago Kent Lamont told me that the cylinder on the S&W Model 29 44 Magnum expanded .001" at each shot using factory loads.

[Old Savage]

Seems that would vary between barrels.

[Bryan Austin]

How do those strain-gauges really work.......

It took me nearly 10 years to finally purchase one. I could not find much information about them, not many reviews etc. Only saw two videos with folks using them so I just had my doubts. I didn't really doubt the program as much as I doubted my ability to operate one and the results be accurate.

I called and talked to Jim at Pressuretrace II many times. The problem I had was finding a firearm to attach the gauge....I couldn't! Because of the placement required for the 44-40, leveractions are a no go as well as revolvers. I needed a single shot and just could't find one. I finally decided to build one. Rifle barrels still wouldn't work because the the threaded end and I didn't want to destroy a perfectly good barrel. I contacted MGM barrels and they cut me one. It is nothing more than a standard barrel chambered for the 44-40 cartridge. For chamber technical information with honest test barrels, check here: https://curtisshawk21.wixsite.com/44cen ... re-testing
I wanted a thick barrel so I could test high pressure loads but it is too thick for smooth "through" readings but the pressure curve is fine. I need to machine it down some but it really isn't needed just for the chamber pressures.

I know this didn't answer your questions but Jim would be perfectly happy explaining it to you by phone.
https://www.shootingsoftware.com/pressure.htm

Here is another short article
https://www.shootingsoftware.com/ftp/In ... 0rifle.pdf

Someone has added some info on wikipedia
https://en.wikipedia.org/wiki/Small_arm ... re_testing

Sorry I couldn't explain the technicalities.

I have also been in contact with Buffalo Bore. Although they share no "secrets", using their 44-40 "Heavy" as my "control" at 11,300psi has not been met with disapproval. My readings have been consistent showing factory (hunting) loads to be 15% to 25% less than Buffalo Bore's and Cowboy loads being even less than that. Original Black Powder testings have been higher than Buffalo Bore's, but just less than Buffalo Bore with black powder and modern cases. Everything "fits" or "adds-up", being consistent. Even testing Lyman's 49th handloading manuals max loads for High Velocity loads are slightly lower than published, which is expected.

Hope this helps!

.45colt....yeah, I guess I have become a 44-40 lunatic...lol, I really got tired of people criticizing me for shooting "published" loads when the 44-40. The 44-40 is no different than the 45 Colt, 45-70 and a few others when it comes to the ability to load them hotter than standard and be perfectly safe for certain firearms.

[JimT]

The 44-40 is no different than the 45 Colt, 45-70 and a few others when it comes to the ability to load them hotter than standard and be perfectly safe for certain firearms.

There are always those who cannot see anything different than what they have always known.

[crs]

To answer your question "how do you do it"

https://www.shootingsoftware.com/

Pressure Trace II - not expensive and creates neat charts too.

[KWK]

I know technology is really cool and stuff, but I would think the amount of stretch at firing of a rifle barrel at the chamber would be super-small

Strain gauges were developed around WW-II and could be read by the electronics of the era. It's easier with modern electronics, of course.

The ratio of stress (pressure) to strain (elongation) in steel is about 30,000 ksi (30,000,000 psi). The yield strength (permanent deformation) of a decent steel will be over 70 ksi, and the design stress will be no more than half that, call it 30 ksi. At a maximum working load, then, the stretch will be upwards of 30/30,000 or .001 inches per inch of length under that load. A strain gauge might be 1/4" across, so the steel stretches .00025" under the gauge itself. The stretch is indeed small, but frozen in time, you might see it with your eye (if you're near sighted enough) and measure it with a fine micrometer.

On the gauge is a zig-zag of very fine wire. Glued to the barrel, it stretches with it. As it becomes longer, its cross section decreases, and both of these changes increase its electrical resistance. It's this change the electronics can sense, even 80 years ago.

There are many ways to sense pressure in the cartridge, but this is the common one today, and you can apply it to your own guns (if the construction is suitable).

[piller]

Tieing this in with another recent post, steel can stretch under pressure and return to its original shape many times if the pressure is relieved quickly enough. If the pressure is kept on and is strong enough, it can cause steel to permanently deform. That is why SAAMI gives pressure limits. You can go higher than standard pressure in certain firearms without danger, but it is possible to go over the limits of the steel and cause a catastrophe. Caution is not a terrible quality to use when testing the limits. How else would one be able to pass along knowledge gained without scaring off the intended audience?