I know you asked this question 2 months ago, but you might find the stuff in the following website useful:

http://www.chuckhawks.com/458Win.htm

It duplicates the Speer reloading manual suggestions.

Quote

In my experience, SR 4759 powder has proven more accurate and more reliable than IMR 4198 or H4198 in light .458 Mag. loads. And the Speer 400 grain FP bullet has proven a better choice than the 300 grain HP bullets intended for .45-70 loads.

Here are some specifications of interest to .458 Mag. reloaders: bullet diameter .458", maximum COL 3.34", maximum case length 2.50", MAP 53,000 cup.

Here are useful .458 Mag. loads for hunting CXP2 class game in the woods, courtesy of the Speer Reloading Manual, Number 13. 26.0 grains of SR 4759 powder behind a 400 grain Speer flat point bullet will give a MV of 1262 fps, and 30.0 grains of SR 4759 behind the same bullet gives a MV of 1445 fps. These loads were tested in a 24" rifle barrel using Winchester cases and CCI 250 primers.

I load 29.0 grains of SR 4757 for a MV of about 1400 fps with the Speer 400 grain bullet, essentially duplicating the traditional .45-70 factory load. This load is exceptionally accurate in my Browning/FN rifle.


Dan