You will be hard pressed to burn all that powder in a short tube using lighter projectiles. The heavier the slug the more inertia is needed to move it up the tube, therefore enableing the powder a longer time to consume in a shorter barrel. If your looking for speed and less drop, opt for a 45cal with a 400 gr slug. The 45's tend to be inherantly more accurate then 50's, and will fair much better at longer ranges in respect to trajectory and shooter error in judging distance. One consideration that is paramont is to match the slug to the tubes rate of twist, and not to exceed veloscities that will cause the bullet to strip the rifling causing fliers and leading of your barrel.