Frank : YOur maximum charge using a prb will be about 80 grains in that caliber and barrel length. I have clocked a 65 grain load of 3Fg powder in a .45 rifle that has a 37 inch barrel, and we are right up at 1900 fps, or faster. Start there and go up to that 75 grain load. Use a chronograph, and see if you are getting more real velocity for each increment of powder increase, by adding more powder.
Penetration in game is a function of the weight of the ball, and not speed. A round ball loses velocity very quickly, simply because it has a terrible ballistic's coefficient. The ball will weight something around 124 grains. Its more than adequate to kill a deer sized animal out to 100 yds, and probably a bit more, if the shooter can place the ball properly. Expect even that ball to pass through both sides of a deer on a broadside shot that does not strike leg bones. If you are looking for greater penetration, using a round ball, you have to go to a larger caliber to increase the weight of the ball. Using conicals, you can increase the weight of the bullet by increasing its length. A long bullet requires a faster rate of twist to stabilize it in flight, so we are limited on what kind of conicals can even be used in guns made to shoot round balls accurately. In .45 caliber, a ROT of 1:56, or 1:60 is considered desireable for a strictly round ball gun. Most come with a 1:48 ROT, and represent a compromise for shooting Both round ball and short conicals. The slightly faster ROT simply means that the gun might be a little sensitive to any changes in the powder charge loaded.
