I've been experimenting with 8x57 loads, and have found that 52.3 grains of Varget behind a 175 grain Sierra gives 2700 fps and right around 52,000 PSI in my gun.

Your individual results will be different, but this may be helpful for purposes of comparison.

Sounds like a great load - thanks!

Sure, Clark, no trouble.



NIST has a nice little section on their web page that directly addresses indirect calibration. It addresses the issue of how you calibrate a chronograph, and the same rules apply here.



NIST is just plumb out of FPS standards. But, you know the distance between photosensors, and you know the accuracy of the internal clock, so, by the formula speed = distance/time, you know speed to the same precision as you know distance and time, and the chronograph is calibrated, even though you have no FPS standard you can directly compare with.



Strain gages come from the factory calibrated to three significant figures. There are numerous ways to calibrate a strain gage to basic, traceable quantities, such as mass, and it's not very hard. If you know the within lot variation, and know that it is very small, then it is fair to extract a sample from the lot, test it, and apply that value to the other members of the lot.



The next thing you need is a few dimensions. Again, those are easily traceable to NIST, and easily gotten to three significant figures. Ordinary dial calipers are completely adequate for this task.



The next thing you need is a constant that is derived from a ratio of two physical properties of steel. I researched this on the web, and the most definitive statement I could find is that the ratio is "remarkably constant", even across different types of steel. That number is known to at least three significant figures.



In the bad old days, getting an amplifier that would "hold zero" to within a couple of percent was a pretty neat trick. You had to resort to things like chopper stabilization, and such. In modern electronics, getting something that is linear and stable to within .1% is as simple as a $7 instrumentation amp chip.



The final thing you need to know is that you are stressing the steel well within its elastic limit. Then your are sure that Hooke's Law applies, stress is proportional to strain. In fact, under the circumstances, a gun barrel is about as well-behaved spring as you are likely to find.



The math to find the standard error of the system isn't too hard, but it is also not worth doing. If you have a set of input variables that are known to three significant digits, which we do, the output variable will also be good to nearly that, too. In our case, two digits would be ample.



Two digits gets you to within a percent, or about 600 PSI. That is more than good enough. Since all the input variables are much better than that, accuracy is good enough.



If you think about it for a minute, the old CUP system is not calibrated at all, at least not in units of pressure. CUP is strictly a relative system, and it served us reasonably well for many decades. I've spent some time on the phone with Hodgdon's lab, and I doubt that they can tell you with any precision what the standard error of their piezoelectric system is, either. However, they do go to great lengths to make sure it is as small as possible, and not a big factor in evaluating loads. So, for them, the answer is, it's more than good enough.



The strain indicated on the 'scope is directly, mathematically related to the pressure inside the chamber, and all the numbers in the equations relating strain to pressure are known to more than adequate accuracy, so we know pressure to adequate accuracy.



If you want to build your own, you might start with the Burr-Brown INA128. Or, if you have a good Tek scope, about all you need is three resistors, a strain gage, and a 7A22 Diff Amp. Run the gage as one leg of a Wheatstone Bridge. That lets any ambient noise appear as a common mode signal, that the diff amp will largely reject. The equation relating pressure to microstrains is fairly simple. If you really want it, I'm sure I can find it again.