There are soooo many powders that work well in the 45-70, that it is just about a case of see what powder you have on hand, and load it. The old original printing of the Lyman Handbook of Cast Loading, shows a 500 grain gas check bullet, loaded to 1625 feet per second, with Unique! That's a darn skookum load and normally one woudn't think of loading so high, with such a fast powder.
Virtually every powder from Unique and slower, until you get to 4350, which is too slow, will work fine with farily heavy bullets. I have never used 3031 in it, just because I don't have any on hand. But some of the most experienced 45-70 loaders and shooters on here swear by 3031.
In my Marlin 1895 I have used Unique, 2400, 4227, 4198, 4895, 4320 and Norma 203, that I can think of right off the top of my head. Of those, I like 4895 for medium and heavy loading of 420 grain cast bullets, with 4198 a close second, but not as good for heavy loading. However, I have not done as much with 4320 and Norma 203 and one or either, are likely just as good. As well as the tried and true 3031 and likely 4064, besides many of the newer powders. Why not Varget for heavy loads?
It is mainly because of the Springfield Trapdoor that factory loaded 45-70s are loaded light, as shown in the ballistics. I don't take ballistics all that seriously, and I caution not to sell the "anemic" factory loads too short. I have a beautiful Springfield Trapdoor that has been in the family since the early 1930s. An older brother shot a mature elk with it, using a factory loaded 405 grain jacketed bullet. He made a body hit at about a 45 degree angle, hitting it just behind the shoulder. The bullet went right through and out the other side. The elk decided it was time to lie down.
Cast bullets, either flat based or gas check, of the 400 to 420 grain range work so well and so accurate, that I can't see why one would pay a geater price to get jacketed bullets. I don't have experience with bullets heavier than 420, but some sing the praises of the heavy bullets. I tend to think the 420s would go through any animal I hit with them, making a large wound channel.
In short, if you are loading for a strong action, like the Marlin, it is pretty hard to do much wrong. My top loadings are always governed by the whummping I take, and not because of too much pressure in the rifle!
