If you'll look at the Buffalo Bore ammo chart in a Cabela's catalog you'll see those rifles mentioned for their 45-70 magnum loads. Oughta tell you how tough they are.Caleb C said:
Myself...I've shot a few thousand of those big 535 gr. bullets out of my Sharps with IMR 3031 powder. My load chronies at 1600 fps. with the 535 gr. Postell cast bullet and 1550 fps. with the 500 gr. Barnes Triple Shock bullet. Recoil isn't too awful bad cause of the weight of the rifle which is 13 pounds. The steel buttplate will get you more than anything. Anyway...a buddy of mine has the same two rifles you mentioned and wanted the same bullets I shoot out of mine. Course I dropped the powder charge by 6 grs. for those little light rifles. Shoots well out of the Buffalo Classic and the recoil is tolerable...but...those big bullets aren't nice at all out of that little light handi-rifle.
So...when you find out for yourself how tough those rifles are and if you're interested send me a pm and I'll give you a load for them.