Other Danger Will Robinson
There seem to be quite a number of misconceptions that have popped up in this thread, some with potentially hazardous consequences. Aside from the last one, above. I'll forget some, but here are the ones sticking in my mind goes:
Copper crushers are very imprecise in producing actual pressure numbers. That is why their output is given Copper Units of Pressure (CUP), and not PSI, though they once were. After it was figured out the real PSI could vary quite a bit from what the copper slug said it should be, they made the change to the CUP. If you measure three known pressures by the copper crusher method and three by any one of the three Piezo methods (SAMMI center ported chamber, CIP ported neck, or Conformal Piezo Film transducer (considered most accurate, but most expensive because the transducer is destroyed during each shot), the data points will only match vaguely. Sometimes the copper crusher will read too high, most often at lower pressures, and sometimes too low, most often at higher pressures. The Piezo device won't be dead on, either, but it will be closer. To learn more about this, see physicist Denton Bramwell's article, Correlating PSI and CUP, available on RSI's website under Support|Technical Articles.
Interestingly, while SAMMI puts 35,000 PSI as the maximum pressure for the .357 Magnum today, the CIP puts it at 3000 bar, or 43,511 PSI. Why the difference? We have both a lot more liability lawsuits and a lot more older and poorly maintained firearms than the Europeans do. There are a number of older rounds where SAMMI peak pressures have been reduced over time because of this concern, while the CIP maximums don't necessarily follow suit. The discrepancy is usually smaller for modern rounds, like the 40 S&W.
During the famous H.P. White Laboratory tests, the strain gauge was found inferior to the Piezo transducer, but that was awhile ago. Electronics for measuring strain gauges have been improved, so this accuracy ranking has now reversed. Again deferring to Denton Bramwell's calculations, the standard deviation of errors for a modern strain gauge system is now lower than for the Piezo transducers by a good bit, and the Piezo transducer's standard deviation of error remains significantly lower than the copper crusher's.
Same velocity from different lots of factory loads does
not mean same pressure curve. Handloaders use cannister powders which are carefully blended with previously held over faster or slower lots (whichever is needed) to keep their burning rates constant in a standard condition. That is how we handloaders often find our pet loads unchanged from one lot of powder to the next (though this should never be assumed; always back off 10% and work a load up over again for each lot). The ammo factories don't want to pay for all that blending and storing. They use the unadjusted non-cannister grades straight from the factory. They adjust their powder charges for each lot of loads from a powder lot. They try to match velocity performance, but if the pressure is creeping up, they will sacrifice performance to keep a safe pressure margin. They occasionally let a hot batch get past them, but not often.
It is a poor practice, used in many manuals, to assume the same charge will suit all bullets of the same weight and general construction (i.e, jacketed lead core, cast, bronze solid, etcetera). In a large bottleneck case, you can get away with this. In pistol cases small changes in bullet base position affect pressure measurably. In the case of putting a Speer 158 grain JHP over a full load of 296, leaving the load alone, then substituting the Speer Gold Dot of the same weight at the same COL, the pressure is increased about 4000 PSI. That is because the Gold Dot bullet is 0.050" longer and sticks that much further down into the case.
Nonetheless, the above difference is not enough to account for how much Speer changed their recommended 296 load. Underloading 296 like that for a revolver is, according to Hodgdon, putting you at risk of sticking a bullet in the barrel. 296/H110 is hard enough to light that it can squib out when the bullets gets far enough forward for the barrel/cylinder gap to start bleeding gas away. I am beginning to think Speer just isn't very careful with their manual authoring.
As to mimicking the old pressure standard with 296/H110, one of the characteristics of this powder is high velocity for low pressure, relatively speaking. As long as you don't compress the charge, you will get enough behind the 158 Grain Gold Dot in a .357 case to exceed the CIP maximum limit. Fill the case to the depth the bullet base will be seated to, and you should be just fine. How many grains that is depends on the capacity of your particular cases? In a case with 26 grains of water capacity and the Gold Dot seated to 1.590" COL, it is about 16.5 grains of 296/H110. Work up slowly to that, watching for pressure signs and stuck bullets. It should run around 1350-1400 fps from a 6" tube.
Oh, and do not trust case head or pressure ring expansion (either one) as a good pressure indicator. For the third and final time, I'll mention a Denton Bramwell article: It is called CHE, PRE, RIP (case head expansion, pressure ring expansion, rest in peace). He demonstrates just how poor the correlation is. It's been awhile since I read it, but I believe that with a 45,000 PSI real pressure, these indicators might claim you are at over pressure at anywhere from 30,000 PSI to 60,000 PSI. I may have remembered the numbers incorrectly, but it was that kind of error. Big. His articles are
here.