The provided expander in most die sets are for jacketed bullets,RCBS Legacy dies have expanders that are suitable for cast bullets, still not the best choice.
Go to Buffalo Arms.com and purchase a custom expander.I use a .458-.462 expander.This will cover most if not all the various sizes of ".458" cast bullets.Expand the cases just enough to seat the bullet by hand to the first grease groove.Lever guns need a crimp unless hand fed one at a time, I have found that setting the bullet seating die up to not crimp, just seat the bullet to desired depth and crimp as a separate operation.You will have less problems and better accuracy with this method.