Not familiar with the bullets, pretty familiar with a 45-70 and trapdoor’s.
Lots of things to consider, but which bullet are you considering, the solid or the jacketed?
I'm going on a Bison hunt, and I want to use my 45-70 1873 clone. I can't find any data to load the nosler 500 grain jacketed solids in it.
A 500 grain brass solid will be a very long bullet, basically the length of a 600 grain cast lead, so it might not even stabilize. Brass has a density of about 80% lead. So two bullets of the shape shape, the Brass will weigh 80% of the cast.
The Nosler 458 solid is 1.530”, a 500 grain cast is closer to 1.300” depending on the shape. So there will be a lot of case capacity lost. You can add to that poor bullet design for a 45-70 to complicate things.
The pressure/crimp groove on the Nosler appears pretty far back on the bullet. Probably because of the generous free bore of the 458 Lott chamber. The 45-70 is basically “0” free bore. So most likely you will need to seat that bullet even deeper to keep it out of the lands. More case capacity lost.
Now you have a combination of low case capacity, long bearing surface, and hard bullet features that will drive pressures up, in a pressure challenged chamber. Probably resulting in lower than hoped for velocity, in an already velocity challenged cartridge.
The 500 grain partition, will split the difference between the solid and a cast. But again there are design issues. If you are expecting expansion from the partition, adequate velocity probably won’t be achieved.
Accurate powder has data for bullets up to 500 grains with both low and medium pressure loads.