It seems odd that there’s such a big difference between jacketed and cast though?
One of the things you need to understand is that loading manual data represents what the testers felt was going to me the most common or most popular use of the bullet.
One rarely finds data for reduced "cat sneeze" loads in magnum cartridges load data, mostly because the testers figure people who are shooting magnums will want magnum level loads not light loads.
There are three recognized (though not official) load levels for the .45-70.
Standard factory ammo is loaded to the black powder specs even though loaded with smokeless powder and jacketed bullets, Pressure and velocity are safe for the weakest action rifles likely to be encountered, the Trapdoor Springfield.
This level load is also the SAAMI industry standard.
The next level up are loads intended to push the max in Winchester 1886 class rifles, and are not considered safe in Trapdoors.
The 3rd class is max level loads that are safe in Ruger single shots and custom bolt action rifles (usually Mausers). They push the .45-70 to the extreme and are only a few hundred fps below .458 Win mag. These loads are DANGEROUS in Trapdoors and other rifles in that class.
Your Sharps is mechanically stronger than a Trapdoor Springfield, but it is not considered as strong as the Win 1886 or (new) Marlin 1895s, and should not be used with those load levels.
One usually finds jacketed bullet data to be hotter than cast bullet data because the people doing the loading expect people will want to push jacketed bullets faster and while you can push lead bullets considerably faster than black powder speeds, it takes just the right combination of allow and sizing and some other factors to do it without issues. with jacketed slugs, you just load them with more powder and there you go.
