There are quite a few here that shoot cast in their 30-30s, it's just a matter of getting the right sized cast bullet for the bore. Older barrels of the Topper 158 had nice deep rifling, they definately would have been easier to make great cast shooters out of em. With the shallow rifling, I think I'd try a .311" gas checked bullet most likely, but I've not shot any cast in mine. I do shoot cast in my .35 Remington that's a rechambered 357mag, it's extremely accurate with 200gr Mt Baldys.
Tim