Quite a few years ago, I killed a large 8-point whitetail with 50 grains of FFg powder in my .50 longrifle. Distance was about 75 yards and the ball lodged just under the skin on the off-side after breaking the near-side shoulder. These days, I would consider that load a little light for that distance, but I can't really argue with the results. :idunno:
Last year, I killed a small doe out of a tree stand with my Colt Dragoon. I was using 45 grains of FFFg behind a .454 ball and getting about 1100 fps, if I remember correctly. I had limited myself to 25 yards and the deer stopped broadside for a 7 yard shot. This load gave full penetration and the deer only went about 25 yards before piling up.
To make a long story short, small charges of BP can be quite effective, but I believe you should limit your range when using them due to energy and trajectory concerns. But as stated earlier, placement trumps power every time!