T. O'Heir said:
The Hodgdon site Max load is over SAAMI Max pressure too.
No. The Hodgdon load is 40,700 CUP. The SAAMI MAP is 45,000 CUP, or 35,000 psi (see
pages 11 and 19 of the SAAMI standard). Copper crushers register higher pressure numbers than the conformal transducers do in 357 Magnum when shooting the same load. I got this information in a phone conversation with Ken Green about fifteen years ago when he was still SAAMI's Technical Director. To set the standard for the conformal transducer, the member labs were asked to use the same lot of reference ammunition to run pressures in the original copper crusher and in the (then) newer conformal pressure transducer, and the transducer gave 22.2% lower magnitude readings than the copper crusher. In most rifle cartridges it is the other way around, and the conformal transducer reads higher than the copper crusher. But at lower pressure, they can read the same (45-70, for example) or the transducer can read lower (44 Rem. Mag is another example). That difference is an instrumentation artifact. It gives you some sense of how hard it is to get an accurate absolute measurement of cartridge peak pressures, so the SAAMI standard relies on relative measurements (relative to a common reference load) to enable an ammunition manufacturer to stay withing the pressure limits others load the same cartridge to.
T. O'Heir said:
Velocity has nothing to do with measuring or estimating pressure.
It is correct to say you cannot use velocity alone to estimate peak pressures when powders are different. However, it is going too far to say it has nothing to do with it. Indeed, while you cannot measure peak pressure directly by velocity, you can get a darn close measurement of average pressure from the kinetic energy of the bullet from the bullet weight and its measured velocity. Kinetic energy equals the work done to accelerate the bullet and even has the same units of ft-lbs. So, divide the KE by the number of feet the bullet traveled in the barrel (starting from where its base is in the cartridge going forward to the muzzle) and you have the average force in pounds that were applied to its base. Divide that average force in pounds by the cross-sectional area of the bore and you will have the average pressure that was applied to the base of the bullet while it was in the bore.
There are a couple of refining steps needed to subtract the effects of muzzle blast and bore friction on the results, but they come surprisingly close to canceling out (typically within 2%).
So, what can you do with average velocity? Calculate the average pressure, as described above, for an example of commercial load data like Hodgdon's. Look at the ratio of the peak pressure measured to that calculated average pressure and save that number.
Next, use an interior ballistics program like QuickLOAD or Gordon's Reloading Tool (if you are using one of its available powders) and the published data's barrel length to produce a peak pressure and kinetic energy. Calculate the average pressure from the KE, as described above, and note the ratio of that average pressure to the peak pressure calculated. Save that number.
Next, change the program's barrel length to match yours (don't forget to add cylinder length to barrel length for a revolver). The peak pressure will stay the same, but the average pressure you calculate from the kinetic energy will change. Look at the ratio of the calculated average pressure to the programs calculated peak pressure to see how much it changed using your barrel length.
Finally, take the average pressure you calculated for your chronograph results and adjust it in proportion to the effect changing barrel lengths had in the ballistics program and then multiply it by that same average pressure you calculated from your chronograph results to get your estimated peak pressure value. I won't be far off. It's just a lot of steps to go through and to avoid errors it is best to make an Excel worksheet that keeps all the steps in order.