I'm curious, John, how was it established that powder ceases to burn by about 4 inches of bullet travel?
I've read this elsewhere, but it's not obvious to me how it came to be known. To measure it physically is daunting. One not only needs to know pressure accurately, but bullet position in the barrel as well, and one also needs a handle on convective and radiative heat losses to the barrel and chamber.
It seems agreed that peak pressure occurs by about 4 inches of travel, but that does not necessarily imply gas production has ceased by then.
If established, this could explain why the QuickLoad simulator can underestimate peak pressure. For a 180 gn bullet in a .30-06, it estimates even a powder as fast as Re-15 is only about half burned by the point of peak pressure, and that a 90% burn comes about 9 inches down the barrel. It's mathematical estimations are hardly exact, but to track as well as it does the actual trends of internal ballistics over such a wide range of cartridges, I wouldn't expect it to be in gross error.
Regardless, I find nothing to contradict the claim that with legal length barrels, barrel length does not select optimum powder speed.
Karl
