There are always better options but do some reading on “expansion” and “energy”
They’re not the best measures of a cartridges performance.
Mass doesn’t diminish. Momentum baby. Check out garret cartridge and see their performance tests. Penetration and wound mass are where it’s at.
I’ll argue that the only downsides of 45-70 are the trajectory and the recoil depending on loads.
If you have to hunt antelope at 400 yards it might not be the best choice for thin skinned game that won’t let you get close. A 6.5 Grendel will do it.
I know I could make that shot with a 45-70 no problem though.
I’m not an MD nor a ballistician, but I’ve done my fair share of reading on terminal ballistics and wound profiles.
Expansion(or fragmentation) and energy aren’t the only measures of performance of a bullet, but they are the best for predicting how much larger a permanent wound cavity will be, beyond simply the diameter of the bullet.
Energy gives you wounding potential (whether that energy is primarily derived from velocity, mass, or both). Expansion and fragmentation can damage tissue with lacerations, but primarily they are used to dump the energy into the target and create enough tissue stretch that it tears. The reason why pistol bullets and slow rifle bullets don’t create enough of a temporary stretch to effectively tear tissue is because they don’t have enough energy to impart into the tissue.
People claim that pistol bullets don’t achieve large enough temporary stretch cavities because of their velocity. That’s kind of true, but not totally accurate. It’s because of their low energy, which is a result of the low velocity. And this is why really massive, but somewhat slow (1,500 fps) rifle bullets can still cause permanent wound cavities that are larger than the diameter of the bullet.
The more energy you have, the more you can increase expansion/fragmentation to achieve the maximum amount of energy transfer into the tissue while still meeting the penetration requirements. Without some form of destabilization/expansion/fragmentation/large meplat in the tissue to transfer the energy, even really fast bullets could just ice pick through.
Now, regarding penetration, yes mass is key to penetration. As is bullet hardness and construction, target hardness and density. You mention momentum, but momentum requires both mass
and velocity. And momentum goes down as the velocity drops due to terrible ballistic coefficients.
Yes, 45-70 with heavy hard cast bullets could be used on large African game, but who among us is doing that? And yes, it’s good for smashing through a grizzly skull, but any of the modern cartridges I mentioned can do that too.