Gary Reeder
first off you are comparing apples & chipmunks
Monday, August 20, 2018, 22:23

The 500 S&W is a close range thumper whereas the 45-70 is capable of longer ranges.
The 500 S&W, out of an 8" revolver barrel will push a 350- grain bullet at 2000. The 400 grain at about 1800. The 500 grain at around 1500 fps. Out of a rifle with no barrel cylinder gap plus a 20" barrel you can add around 500 fps or a bit more to those figures.

A 45-70 in a Marlin lever gun will run a 300 grain bullet at 2200.
A 350 grain at almost 2000.
a 400 grain at 1800 fps.
Going by those figures the 500 will edge the 45-70 by a little bit.
These figures are for a 45-70 Marlin and out of a reloading manual. Your figures might run a bit more or a bit less.