If you are using black powder then the flight path performance is pretty simple as only a small part, if any of your shot will be supersonic. Just copy somebody that has shot the same load with the same muzzle velocity and your pattern work is already done. Black or nitro makes little difference. If you are following the old rule of 96 the expansion of shot and the decay of energy is what we call straight line. Shoot a pattern at 30 yds measure it, then measure your barrel diameter the distances between those two points will be proportional or close enough to proportional and will depend more on the wind and shooting consistency.
