This 45-70 gambit I deal with in my shop all the time. The factory loads are very low pressure so hopefully a person will not blow up their trap door Springfield and sue the ammo maker. However this hole thing comer down to basic phyisics. To push a givin bullet weight/ diameter at a spicific velosity you need a certian pressure. Yes you can push a 500 grain bullet in the 45/70 to .458 Winchester velosities, even higher but your pressures go up accordingly. A 45/70 loaded to Lazzeroni pressures will beat out the .458 but 70,000 PSI is a bit much don't you think. The bottom line on this is that it takes a given pressure to push a given weight bullet at a given speed, the only way around this is to increase case capisity so we burn more powder at lower pressures to reach that bullet speed. This is how the .450 # 2 NE can reach 2150 with a 480n grain bullet with only 25,000 C.U.P or less. On the other hand you have to burn more powder to get there.