First off, let me compliment you on a great gun. I have some 54 cal. Lymans. They are real shooters. The best way to increase your velocity is to switch powders. I used Shockey's Gold for a couple of years and it is the lowest velocity powder. This year I used Triple 7 . If you try either Pyrodex or triple 7 you will gain some velocity. Another thing as far as a deer load is to switch to a conical or ball-et. If yours a a 1:60 twist I have had good luck in stabilizing a maxi ball with 110 gr powder. It will dramatically increase your knock down power.
Then again at 75 yards the 50 cal round ball will do the job with 80 gr.of powder