Possibly you are having trouble because 2400 may not be a good choice for that cartridge and projectile weight.
In 4 decades I have never used 2400 so I will not be much help
I have gotten the best results with my 510 grain wadcutters using AA1680. You can load 46 grains and not exceed pressure limits (getting close) while the projectile hits around 1700 FPS depending on which barrel length you are firing.
I know you are suggesting 2400 but a look at what Hodgdon has to offer in your bullet weight gives you multiple choices in powder use if the Alliant offerings don't work out.
Karl
500gr XTP Mag-32 grains (case fill is about 88%) produces about 55kpsi and 1425 fps- would consider max for this bullet
(tested with 8" pressure barrel)
as suggested below 1680 a better choice
As a side note Vhit N110 produces good results with lead bullets across the entire weight range (300-720 gr.) in 500 S&W.
100% case fill with any lead bullet pressure will not exceed 52kpsi and produce reasonable velocity. All loading's will shoot very clean.