Dane - I don't see any reason why not - you must slug the bore of the .500 to see what size the cast bullets need to be. They should be sized or expanded to .001 to .002" larger than the groove diameter. RCBS makes an excellent 550gr. bullet (1/2 pure/1/2 WW metal) That bullet was exceptionally accurate in my .50 Alaskan Rolling Block up to 2,000fps.

To slug the bore, easiest is to use a .54 round ball, pure lead, and just pount/tap one into each of the muzzles, then poke it out onto a soft cloth. Measure the largest diameter of the 'slug' and that's the groove diamter you have to use. Once the 'slg' is almost flat on the muzzle, I use a piece of Ramin or maple or hickory dowel to push/pound it down into the muzzle, then poke it back out using a rod from the other end. It's easy to do. A 1" michrometer is better/more accurate than calipers, but calipers will work just fine.

Modern Alox/beeswax lubed can be used, either in a lube sizer or hand lubed. You may find the bullets don't need to be sized and you can shoot them as-cast. I prefer a mix of beeswax and vaseline for lube. 60BW/40%Vas. This lube owrks perfectly for black powder as well as smokeless powders, BTW and prevents leading to well over 2,000fps in smokeless loads. I'd use straight WW metal for the melt. You may add some tin if you want - some say it helps casting. I've never in 35 years had any problem with straight WW metal. Make sure there are no ZINC WW in the lead, nor any glue-on WW's as they are pure lead and will soften the alloy. Zinc WW in the melt will destroy the whole mix and can ruin a pot as well. Zinc in the mix makes it very difficult to cast good bullets.

Good luck. Lyman, Lee and RCBS all have relatively cheap moulds for 450 to 550gr. bullets. The RCBS 550gr. (they may call it a 500gr. or 520gr.) is the most accurate in my findings. The Lyman 520gr. is the second most accurate. I haven't tried the RCBS 450gr. FN- it might be the best overall - I don't know.