I have no experience with this like Hawg and others, but think you would also have the option of using a smaller bullet with a sabot. That would be more fussy because you would deal with the sabots, but may work depending on your barrel twist and desired bullet weight.
I Googled 'Gonic .458 bullet' and came up with a list of results.
https://www.harvestermuzzleloading.c...-for-your-bore
Harvester handles bullets and sabots. I know there others on this forum with actual experience that I don't have. You might do a search on all these Firing Line forums ( an overall search) for past posts. Good shooting.