Jani,
Don't have a 24 bore but do have a 20 bore (ga.) rifled SXS I load for. I have used round balls with moderage success but you might want to try either Mini or Maxi balls in 58 caliber, I would suggest the Mini Ball first since the skirt will expand to help grab the rifling and impart spin to the bullet. I used a thin over shot wad and roll crimped, using fiber wad(s) under the round ball to get the height needed with one over powder wad.
My gun is a bit newer than yours so I am able to shoot Sabots in it and found one that shot very well which I use but I would not suggest using these in yours as they are higher pressure.
Enjoy your combo gunwhen it arrives....
WBLDon
