Now I wont pretend to know all the 'whys and wherefores' of this terminal ballistics stuff; a lot of the time I think I come at this from maybe somewhere 'outside the box' with a different perspective, and hopefully some 'common sense' that provides me some insights.
So.......It sorta seems to me that after the muzzle exit velo. the bullet is slowing down, how much between muzzle and critter will be relatively easy to calc., then it hits the critter and is slowed down at an even faster rate, how much depending upon just what it has to pass through from skin entry on (and lets assume a non deforming projectile). Thus everything is acting to slow that bullet, and as penetration at first is a function of mass and velocity, and its rapidly slowing, the pen. capability is probably acting in reverse proportion as to why we try to make bullets go faster in the first place. At some point the mass (ie, momentum) must be the prime motivator rather than the decreasing velo. and a heavy bullet will go farther than a lighter one.
The basic physics are still at work here but the rapidly decreasing velocity curve is losing the battle while the weight is carrying on, just trudging on through bone and muscle.
Isnt this somewhat analogous to the lead shot vs steel shot debate in waterfowling?
I've not come to a conclusion on this yet, but looking at lots of calculations with a ballistics calculator, it does seem to me that the momentum factor of the heavier bullet starts to come into play as retained energy at longer distances.
Momentum = mass * velocity
but
Energy = mass * velocity squared
So velocity increases clearly jump the energy number up... but if mass is low... retained energy falls off faster.
So what I noticed in all the tables I generated is... the 30 cal bullet may start out with more energy than the 45 cal heavy.... but down the range there comes a point where the 30 cal drops down to the 45's level... and then drops energy faster....
Again the cost being a more rounded trajectory of course. But in some cases I ran, the 45 cal would start with less energy at the muzzle, but have more left at 300 yards.
This was using the Lee 500's BC of .443, which is pretty high compared to cast bullets in 30 cal on average. The 405's weren't quite as good in the momentum respect. Less mass, lower BC.
Anyway, nice as high velocity is, for 45-70 seems to me the 405 an 500's at traditional velocities ought to get the job done... 300 or 350s moving faster if you want that flatter trajectory.
About the only thing that caught my eye was the other end... trying to shoot round balls in them... the little 40 grain 30 cal round balls would let you drop down to .22 levels a lot easuer than the 145 grain 45 cal. round balls.
Of course my goals may be odd. Basically trying to not have to use jacketed bullets. The less components I have to buy the happier I am. Of course since powder and primers are practically always a buy rather than make proposition, I guess that's an irrational way of thinking.
