First slug your barrel to find the groove diameter. Cast bullets have to be at groove diameter or larger. Unfortunately Marlins, I've slugged over a dozen, seem to be made on the large side. This is why Marlin microgroove barrels got a bad rap for use with cast bullets. My .45-70 marlin was. 4592", obviously. 458" bullets were a disappointment. Instead I made a mould that cast out at. 461" and sized to. 460" and all was good. A couple of other owners took my advice to use bullets as cast, without sizing, and that worked out well too.