Dunno either, and can only guess at what Steve's point is about Homer Powley and the 4-to-1 Rule.
For those who aren't familiar with it, the 4-to-1 Rule is that any increase or decrease in the powder capacity of rifle cartridges of the same caliber results in about 1/4 that amount of potential velocity, with the same bullet at the same basic pressure. One example I've used frequently is the .300 RUM and .308 Winchester: The .300 RUM has just about twice the powder capacity as the .308, but does NOT produce twice as much velocity--a 100% increase. Instead the velocity increase, with the same bullet at the same pressure, is about about 25%, which is 1/4 of 100%. This rule does not apply to the SAME powder, but the powders producing the highest velocities in each cartridge.
I am quite familiar with Homer Powley's slide-rule calculator, having purchased one around 40 years ago--and still have it in my bookshelves, packaged in a manila envelope with all the accompanying literature. Can't figure out what the 4-to-1 Rule has to do with any of Powley's basic rules, except that it's another phenomenon derived by crunching the numbers from LOTS of pressure-tested data.
The only similar rule I can remember from Powley is that pressure with single-based powders increases at twice the rate of velocity--which might be called the 2-to-1 Rule. But that was derived from copper-crusher pressure data, not the piezo-electronic data used by the best pressure-labs today, which shows the velocity-pressure relationship isn't quite that simple, even with single-based powders. It varies even more with double-based powders.
Steve also seems to be arguing that pressure-judging by primer appearance works very well. Would love to hear if he can supply some data. I seem to remember Denton mentioning that he'd tried it, and found no correlation.
MD remembers correctly.
I wanted to do an article on flatness vs pressure. So I carefully prepped and fired a batch of ammunition with various powder charges, using primers from the same box. Under a microscope, I arranged them from roundest to flattest. I could see no detectable correlation between flatness and powder charge.
