Certainly, at level of accuracy needed and acheived for accurate long range shooting from rifled cannons, even shotgun powders are too diverse to allow similarly accurate equations. However, I suspect that a usefull parametered set of equations could be derived for popular types of shotgun powders. And I would think that powder makers know something about this, really, they have to. Presume that there is no incentive for them to make even generalized informations available to reloaders, hence we never see that information.
One could derive some rather usefull data simply by having a few popular low-pressure loads P&V tested at a range of powder temperatures, say from +40C down to -20C. With max of 6-7 data points over this range of temperatures, at even ten dollars per shot, would cost much less than what most folks on this board spend on a single gun, or even "inexpensive", 300 dollar repairs. With some smart decisions, a very informative testing program could be designed.
Niklas
