I shoot a CVA Optima Pro in .50 cal with 100 gr of Pyrodex (2 - 50 gr pellets) behind a 245 gr Powerbelt HP. It shoots great and although it will handle 150 grains, I talked with a tech rep at CVA and he said 100 is all you need. The .45 will give you a bit flatter traj. and it seems that what the "experts" on TV are using with the Powerbelts.
But I'm just a bit fond of the .50. Maybe from a nostalgic standpoint. My flintlocks or Optima have yet to let me down. And I do like the break open action on it. And the price is right. I put a 3 - 9X scope on it just because I had it laying around. But a straight 4X would probably be pleanty as you shouldn't be shooting at game over 150 yds.