I would say that your best bet would be to use tin (Sn) or a alloy of such. A few years ago I hit every flea market, garage sale etc around and bought dented unwanted pewter items. Pewter is an alloy consisting mostly of tin. Antique tin did contain lead but they stopped adding that to it over a hundred years ago. It melts at 450 degrees a little less than lead at 621. It is easily cast into round balls. Here in Nanny York historical sites sell cast tin round balls as replicas of lead round balls as selling lead ones would be dangerous as children might eat them.
Tin weighs 35% less than lead.
This means tin has 35% less kinetic energy. A tin ball will peter out much faster than a lead ball. Penetration will be much less with a tin ball as compared to a lead ball.
Effective killing distances will be greatly reduced when a .50 caliber tin ball is pushed using a standard powder charge of, say, 80 grains.
But, there's always a but, with tin weighing 35% less it means you can bump up your powder charge considerably. With zero risk of overcharging the firearm.
Not sure if it's exactly linear, but I would increase my 80 Grain charge 35% and bump it up to 110 grains. This would make for a screaming fast tin ball. It would still bleed off energy quickly, but it would give you a fighting chance at reasonable hunting distances.
PS: Tin balls in a cap n ball pistol would be a terrible idea. Especially for hunting. In a rifle you have the space to increase your powder charge. A cap n ball pistol has limited powder space. Hunting with a tin ball fired from a revolver would be unethical. Unless you have a deer sitting in your lap at the time you pull the trigger.