Consider how little volume is in the shell combustion chamber to start with, how little more is added by the wad moving one inch (about the movement at pressure peak), and you begin to see the expansion ratio. Yep, the pressure drop is due to expansion. We would achieve some real screamer MV's if the pressure stayed up; Pressure times wad base area equals the force on the shot load and acceleration of the shot load equals force divided by mass. Expansion of a gas is usually associated with a temperature drop; both get accounted for by the pressure drop.
I doubt that slow burning powders have a lot of influence on muzzle pressure; some, but not a lot. If they did, I think we could use them more to tune patterning/choke effect much more that we are currently able to do.
