Further to Dan's post, if you shorten .45/70 brass to 2", the Speer 350gr. bullet can be used. Shortening the brass allows crimping in the small groove and gives proper ctg. length for feeding in the Marlins. I'm now loading the 350gr. Speers in my .458 2" bolt gun for hunting though as yet, I haven't shot anything with them. They have jackets over .060" thick and should be very good penetrators and since they are designed for the .458 mag., are supposed to need 1,900fps for expansion, they should make super bear bullets for close range. The Marlin 22" barrel can get about 2,100fps (or more) with them and that's just great. I-too have difficulty with the 'guide' gun's short barrels, but a buddies stainless .45/70 18-1/2" guide gun gets 2,007fps with 400gr. Barnes flat noses - 49.0gr. Reloader #7, straight out of Speer's book and no pressure signs at all.
; My own preference for a camp gun is a 12 bore with solid round balls loaded to 1,550fps, but usually leave it at home and have only the .458 in camp. For bears in camp, I have 560gr. cast bullets loaded to 1,850fps.