If one plans on keeping things within about 400 yds, then a Gov 405 slug, a card wad, and 70 grains of BP. With new cartridges, one may not be able to get 70 grains in there. They expand on the first firing, and one can typically get a few more grains of powder in. One wants zero air space between his powder and bullet base.
One may get even more powder in if he uses a drop tube and a compression die.
If one wants to reach to extreme ranges, then step up to about a 530+ grain slug.
