To chronograph shot loads, make a thick shot cup sleeve, closing both ends over the shot. This will make a " slug " out of any charge of shot you want to shoot, allowing it to pass over the screens as one projectile, and give you accurate velocity readings. It also keeps you from damaging the Chronograph.
The secret to using black powder and shot is to start the shot out under 1100 fps. That eliminates the sound barrier as a disrupter or your pattern. In an open cylinder bore gun, that leaves only the shape of the pellets when they leave the barrel that affects the pattern size, and number of pellets in the pattern down range.
To deliver more energy on your target, use a heavier pellet size than what you are used to using out of a choked gun using cartridges, and smokeless powder. Take advantage of the fact that BP does not created the pressure, nor does it slam the base of the shot load like smokeless powder does. Now all you have to figure out is how to get the shot out the barrel without rubbing the outer pellets against the walls of the barrel, so that flats are put on all those pellets, and you have a pretty dense column of shot headed down range.