RE: 50 cal. thompson center
chuckell.. that's an open question. If you want to shoot roundball and the barrel is 1-48 try 90 grains and a patched ball. If you want to shoot conicals then you might want to lower that a little and start at 70 grains.
All rifles are different. Granted they are close to how they shoot, but to say.. this many grains and this bullet will shoot well.. is impossible. But those should be good starting points.