An inexpensive mold you might try would be the Lee 90991 for the .500 S&W. It casts at .501 or so and would drop about a460 grain bullet in the purest lead you can lay hands on. It's a gas-check design so not ideal but there have been reports of excellent results in some rifles. And honing to enlarge the cavity is a possibility. Aside from that and a couple of other .500S&W molds that are much more expensive, you would need to take a big jump in bullet size to .515 which would require a two-step sizing operation and would still look "skinned". The truth is there just are not very many reasonably priced, off the shelf, molds for .50 caliber muzzleloaders. Those that are available are mostly minnie, maxi, and REALs and are tapered conicals. I'verepeatedly askedLEE to put out a good mold that would drop a .505 modern conical design that could then be sized to fit most any .50 - but so far no luck.

http://www.leeprecision.com/cgi/catalog/browse.cgi?1216606114.275=/html/catalog/bullmol2.html