The Speer manual lists several loads for the 45-70 for use in bolt and Ruger #1 rifles in the 2200 FPS neighborhood.. If your gun is up to these, they are more than sufficient for anything on the North American Continent. I believe they list loads for the 350 gr. hot core bonded bullet and the 400 gr flat nose. The 350 gr supposedly has a much thicker jacket than the 400 and penetrates quite a bit better, and is Speer's recommendation for bear. Take your pick. Have also read several articles on hunting with the 45-70 with flat nose cast bullets (I shoot them in my .458 at about 1500 FPS). The consensus seems to be that the knock down power is directly proportional to the frontal area of the bullet (a wadcutter shaped bullet would have the maximum frontal area, but not practical). In my opinion, the 500 gr bullet is probably overkill, even for a big kodiak or polar bear. I would suspect that if you had to make a 150 yard shot, the trajectory of the 500 grainer at 45-70 velocities could become difficult to manage.
