YOu can produce the same velocity, assuming your barrels are long enough, with both a .45 and a .50 caliber MLer. However, the recoil with the heavier ball begins to affect the shooter sooner as the velocity goes up.
Remember that the ballistic's coefficient for any round ball is -- well----terrible! :shocked2: Any high velocity at the Muzzle is quickly being lost as the ball travels down range. If your deer is within 70 yards, a .45 cal. PRB will do just fine and there is no reason to try to push it over 2000 fps. for such a shot.
In fact, when a deer is shot with a high speed PRB at close range, the ball actually will tend to penetrate less, and may even veer off to one side or another, rather than travel in a straight line, simply because the ball is spinning too fast at impact, and, being made of soft lead, mushrooms quickly at those high velocities. You have to learn an entirely different way of thinking about how a ball or bullet delivers a killing blow to game when you shoot traditional MLers with PRBs, when your prior experience, training, and education has been using modern smokeless powder cartridges shooting copper jacketed, expanding bullets.