Of course, getting back into the realm of maximum sanity, Nosler 5th shows a maximum load (under their "strong actions only" heading for the .45-70) of 53.0 grains of RL-7 under their 300 grain JSP/Partition. QL gives that a pressure estimate of 40kPSI.
Let's play there, shall we?
Extrapolating out the same 40k MAX pressure whilst extending the case to 2.4" for a COAL of 2.85", you can run the same RL-7 up HYPOTHETICALLY to 61.0 grains. Velocity estimate there is 2430 fps, with ME of 3940 ft/lbs. With an estimated 300 grain bullet BC of an even .200, you have a 260 yard MPBR and a zero range of 225 yards. At 250 yards, you're retaining 1495 ft/lbs.
Use a 425 grain bullet at a nominal .300 BC (easily done, or bested), load to the same COAL, and HYPOTHETICAL pressure of 40k in the 2.1" case, and you're at 2030 fps, with an ME of 3890 ft/lbs. You have a zero range of 205 yards, and an MPBR of 240 yards (again, that amazing 20 yard difference...). At 250 yards, you're at 1995 ft/lbs.
Even by claiming the MPBR and energy numbers as a justification to try to hot rod the .45-70 "extended" that you're running, the numbers themselves show otherwise as to the best option.
