Video: Load Development with PressureTrace

[denton]

Here's a short video I made last week, showing how I work up loads using the PressureTrace and a chronograph. Hope you enjoy it.

http://www.youtube.com/watch?v=9phezChbKRI

 

[Clark]

Actual pressure is a distraction from monitoring the weak link in the system, the brass.
Pressures are registered to maintain margins for the brass.
But in an individual hand loader situation, actual pressure adds layers of errors in decision making for load development.

I measure 2675 fps 220 gr Sierra over chronograph with a $50 Mauser made in 1903.

That is working up while watching the brass, finding the threshold of long brass life, and backing off a safety margin to assure long brass life across a list of variables.

You cannot do that well when monitoring pressure, because you don't know what pressure to look for.

In engineering, there are engineers who can find work, but need supervision. There must be a chief who can divine and define the real goals.

 

[denton]

Just because your brass fails, it doesn't mean that your steel won't.

Most firearms are built with a generous safety margin. 99.99% of the time, you don't need all that margin. The other .01% of the time you get something like a case head separation and need it rather badly. If you have foolishly piddled away your safety margin, it won't be there when you need it.

Of course, you could probably shoot for a long time and not need it. Your firearms, your fingers and eyes, your choice. If you don't happen to provoke the demon, the illusion is that he isn't there at all.

Good engineering principles teach us that failure rates rise exponentially with linear increases in stress. If you properly design a firearm, and operate it so that stress is in the ideal part of the stress-strain curve, you'll enjoy long firearm life and you'll have design margin when you need it.

Or not, if you choose otherwise.

 

[nofishbob]

Very interesting, Denton, thanks for posting this.

How do you obtain pressure units from a strain gauge? It seems that the variables of the thickness and configuration of each rifle's chamber would complicate getting a reading that could be compared with other rifles.

Perhaps there is an easy way around this that I am not aware of.

Bob

 

[denton]

You get two forms of data out.

One is the number of microstrains that the steel in the barrel experiences. You want that number to stay in a comfortable operating region of the stress-strain curve for steel.

The other number you get is chamber pressure, which is derived from the microstrain number. If you know the ID and OD of the barrel at the point where the gauge is attached, and the thickness of the brass, and the physical properties of barrel steel and case brass, you can calculate pressure using the Hoop Strain Equation.

As you have noted, the calculation for different barrels involves different thicknesses of steel. Fortunately, the physical properties of barrel steels are quite close to being the same.

 

[Clark]

Pressures are registered to maintain margins for the brass, not the thick steel of a Mauser.

 

[LiveLife]

The Rifleman's Journal did some pressure testing for their large and small primer study using a strain gauge - http://riflemansjournal.blogspot.com/2009/06/primers-small-rifle-primer-study.html

Here are some excerpts from their study in regards to testing equipment setup, accuracy of data collected and interpretation:

With our test equipment in place, we began the pressure and velocity testing portion of this test. We first fired the reference ammunition provided by Lapua in order to verify the calibration of our equipment, an Oehler 43 Personal Ballistics Laboratory system. Our pressure readings were within 100 PSI of Lapua’s specification; this was determined to be well within acceptable limits. Having preliminarily established the accuracy of the system, and due to the relatively limited amount of the factory reference ammunition, we loaded a reasonably large amount of our own reference ammunition to approximate the factory pressure and velocity. This was done using new Lapua brass, Lapua 105 grain bullets and VihtaVuori powder. One of the test primers, which had previously shown a very high level of consistency, was selected to be used in our reference load. All primers were seated with the Sinclair tool which has proven to be the best tool for this purpose.

To the extent possible, SAAMI testing procedures were used as a guide for our testing. However, there are many variances from those procedures. Most notably our use of a strain gauge pressure measuring system (Oehler 43) rather than the SAAMI standard copper crusher or piezoelectric transducer method as both of these are beyond the scope of our budget and facilities. Further, as the 6BR cartridge is not covered by SAAMI standards, the exact location selected for placement of the strain gauge was based on our understanding of the principles involved and the study of similar cartridge specifications in the SAAMI guide. Despite these variances, we believe the results obtained to be reliable and repeatable. (See: Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Rifle Sporting Ammunition for the Use of Commercial Manufacturers, American National Standards Institute 1992, 1999).

Firing our reference ammunition showed it to compare quite favorably with the factory product in terms of consistency and met our goal of having a larger supply of reference/calibration ammunition. While our reference load was slightly lower in pressure and higher in velocity than the factory reference ammunition, it was within CIP specifications for the 6BR Norma and produced very consistent pressures. This difference is most likely attributable to our unavoidable use of a canister grade powder as the factory powder is not available to reloaders. The factory reference ammunition was subsequently used to check the equipment prior to each session and our handloaded reference ammunition was used before and after each test to ensure that external factors such as ambient temperature were properly accounted for.

All testing for this article was done with a Borden action with new Krieger 1:8" twist barrel, 30" long chambered in 6BR by Lester Bruno and stocked in a MAK Enterprises Tubegun stock. The scope used was a Leupold BR 24 with the Tucker coil spring modification. Accuracy tests conducted prior to the primer tests showed this to be a very accurate rifle for its intended purpose: prone shooting at distances of 300 meters to 600 yards. The load used for the tests is the same as our reference load except for the primers, these being the only variable element. Each test firing consists of ten shots. (More at link)

 

[Clark]

The max pressure for 6mmBR is not determined by the brass, but by the threshold of primer piercing.
This is because large Boxer primer pockets can get loose in the Mauser case head, but with a small primer pocket, the primer is the weak link.

With a firing pin bushing job, like one by Gre-Tan, can increase the pressure threshold.

I have converted a few rifles to 6mmBR, and not all firing pin - firing pin hole fits are the same.

To get the thickest cup, usually a CCI450 small rifle magnum primer is used.

 

[LiveLife]

Hmmm ... Clark, I posted the excerpts to reference strain gauge testing method used and not limitations of brass/barrel/primer cup but that's why I like THR ... this newbie .308 reloader is learning something new everyday!

 

[blarby]

Very nicely done, Denton.

Makes the work some of us do look downright barbaric by comparison.