Sir - using a ballistic calculator, I get the following muzzle energy readings for the two most popular bullet weights for the .45-70 Government cartridge -
300 (Trapdoor/Sharps) Lead PB 1,597 ft/s (487 m/s) 1,699 ft·lbf (2,304 J)
405 (Trapdoor/Sharps) Lead FN 1,394 ft/s (425 m/s) 1,748 ft·lbf (2,370 J)
300 (Standard - modern action) JHP 2,069 ft/s (631 m/s) 2,852 ft·lbf (3,867 J)
300 (Strong - modern action) JHP 2,275 ft/s (693 m/s) 3,449 ft·lbf (4,676 J)
How did you arrive at the figures that you quote, please?
tac