Guys - 6 drams should be fun in an 8 pound 4 bore gun. My 14 bore rifle was 'fun' with 6 drams in a 9 pound gun with only a 482gr. ball. Not many black powder shooters would shoot it, let alone fire a second shot for those who did. I thought it was pleasant.

Swaging or sizing a ball will not reduce it's weight as the lead is pressed inside the ball rather than shaved off. Shaving will, of course, reduce it's weight.

For large balls the the filling hole in the mould must be large enough to allow very rapid filling of the mould. Most store-bought moulds have too-small holes for making 600 or 700gr. balls or bullets, let alone 1,500gr. ones.

A clean down-spout in a bottom pound is absolutely necessary for a fast fill for heavy balls or bullets. When clean, my Lee 20 pound pot pours plenty fast, but if the lead and the filling spout in the bottom aren't really clean, dross being pulled into the vent will quickly plug the hole to reduce flow a bit. No problem with small 500gr. bullets or balls but deadly for the really big ones. A dipper with enlarged hole will work well and most likely end up being specific to large bore guns. I would dipp the big ones myself.
I have a very large pot, actually only about 6" across and about 8" deep, that holds an enormous amount of lead. I poured my 50 pound river anchor in one go with it. I think the capacity is around 70 pounds. It is simply a section of pipe with a plate welded onto the bottom, then ground round. Any friends in a welding or muffler shop? Scrap metal yards will have pieces of pipe. Oxy-acetylene will work, but a stinger is faster and probably better. For fast melting jobs of large quantities of lead, I use bricks for a support and channel for the Tiger Torch attached to a 20 pound propane bottle. It will melt 60 pounds of WW in about 5 min. This is how I reduce WW for pouring into ingots. The tiger torch isn't necessary for casting as a camp stove will work - better is a camp stove with the regulator removed - but we won't go there. Mine melts a full - big pot in 20 min. That's propane at 16psi instead of 2 psi.