I was hoping to put an upper bound on the energy of a projectile from a given weight of BP. The amount of energy in BP is not dependent upon its grain size.

The maths are correct, unfortunately the (in)efficiency of realising that energy makes the number useless.

The rate of deflagration is admittedly different for different granularities and is also dependent upon the peak pressures achieved - the chemical energy contained therein is a constant by weight.