I have two Browning BPCR's - a standard BPCR in 40/65 and a "Creedmoor" in 45/90. Strength is not an issue, with black OR smokeless. I have never shot anything but black powder in either rifle. I am not prejudiced, I just bought these rifles to shoot black and that's what I do. Both, I have to say, are superior rifles.
If I wanted to shoot smokeless in the 45/70, I would just simply get Ken Water's books, "Pet Loads" published by Wolfe Publications and read the series he has on the 45/70 including ratings on the strength of various rifles. The Browning would fall into the "strong" action category.
I have loaded my Marlin 1895 almost entirely with smokeless and find it is an accurate rifle but have only shot it at 100 yards. I have no experience shooting smokeless with five hundred grain bullets for long range use.
The old NRA Reloading Handbook may have some data for you.
I am sure that it can be done. I have my doubts that you will be able to buy really match quality cast bullets from a commercial source. No one will spend the time necessary to produce match grade bullets like you will yourself - at least that has been my experience. However, YMMV.
I have a Marlin model 95 lever action ("new" rifle) and I will tell you, you can safely load the rifle with smokeless that'll make your retinas bulge. The Browning is even stronger. So, your choice of loads can and should be predicated on accuracy and comfort.
I recommend RL-7 for a powerful load. It burns well and due to small grain size measures extremely well. You won't need wads and the powder seems to work very well with black powder calibers. Loading data is available from a variety of sources that show loads with RL-7. The Hornady (4th Edition) lists loads for several different action types (trapdoor, 1895 Marlin, and the Ruger #1). You can use data for the #1 in your Browning if you need VERY powerful loads (with RL-7 you can drive a 350 grain jacketed bullet at 2100 fps). It that doesn't rattle your eyeballs, nothing will
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Frankly, if you are just shooting for fun, use the Trapdoor level loads. Even with trapdoor level loads you can drive a 300 grain bullet from 1300 fps clear up to 1900 fps.
If you are serious about developing smokeless MATCH loads with cast bullets it CAN be done. You will find that with cast bullets, accuracy will be your limitation (you can only drive a cast bullet so fast before losing accuracy). If you are allowed to use gas checks, you can load to greater velocities. However, at the higher velocities and 500 grain and heavier bullets, recoil will definitely rear it's ugly head.
Good luck!
Dale53