The velocity changes on the .45-70 Gov't. are interesting.

from 27" to 13": -267 fps/14" = -19 fps per inch for that huge range.

from 27" to 20": -75 fps/7" = -13 fps per inch

from 20" to 13": -147 fps/7" = -21 fps per inch

from 24" to 22": -4 fps/2" = -2 fps per inch

from 24" to 23": +1 fps/1" = +1 fps per inch of barrel shortening.

That is with 405-gr Remington FNSP and H4198.
The .458 Winchester Magnum might be very similar to that.
Maybe just shift the curve to the left by an inch to allow another inch of barrel
for burning of the bigger charge of powder.
28" to 14" for the .458 WinMag Encore, heh-heh-heh.

Ron aka "Rip" for Riflecrank Internationale Permanente
NRA Life Benefactor and Beneficiary
.458 Winchester Magnum, Magnanimous in Victory
THE WALKING DEAD does so remind me of Democrap voters. Donkeypox.