A 480 grain flat nosed bullet at 1242 fps will shoot through and through both shoulders of a 2200 pound bison bull at 100 yards from a 45-70. DRT. Been there, done that, as well as quite a bit more in both Africa and North America.
You have gotten good advice here--there is no need to drive these bullets over 1300 fps. My bullet of choice is a Lyman 457121PH which has seven grease grooves and keeps most of the bullet inside the case. It cycles through my Marlin Cowboy slick as a whistle. The only problem is that you have to cast them yourself. A real good option is a 502 grain flat nosed bullet from Powder Inc. I have seen it work on elk and hogs, and it is a hammer. Over the years, 37 grains of 3031 has been a good load, giving about 1300 fps with these bullets. An equivalent charge of RL7 works well also. You can reach Powder Inc at (479) 705-0005.
Bullet weight, a good meplat, and inertia are your friends. You don't want or need 1500fps unless you are into self inflicted pain and want a recoil headache. A 45-70 with these loads is a LOT more rifle than people suspect, and gives penetration you will not believe.
