Dave,
My Huntsman loves the 460gr No Excuses! I shoot them with 70-75 grains of 777 in either FFG or FFFG. I also shoot the old White Super Slug in 430 grain.
Did you get the standard ones in .503 or did you ask for the ones in .500? You want the ones in .503. My bore on my Huntsman is .5025 and the .503's are almost a slip fit like the White System. You will find that your gun will shoot better if the barrel has been fouled by either a squib load or a full house load. After that don't even think about swabbing between shots if you are shooting for groups. If you are trying to find a hunting load then you would either hunt with aq fouled bore or find out the POI with a clean bore.
Back off on the select to about 65-70 grains or even 60 and work back up until groups open up. Try 777 if you have it or an FFFG grade of black or Pyrodex P. The big slip fit conicals need a fast powder to obturate the bullet into the rifling. Don't give up on 'em just yet, there is a load they will like if you take the time to find it. Good Luck!