Hi sixshooter,

here are a few load recipes which were working in my Heym Express in 500 A-Square.
Precision was fair to good (minute of plate at 50m), but I´m not a good shot with open sights.

brass: fireformed 460 Weatherby (volume ca. 150grs water, length: 73,3mm)
primer: Winchester large rifle
bullet: 492grs hard cast round nose with small meplat,
bullet dia. .510" (bullet length: 26,0mm)
I single loaded the cartridges, therefore I did not apply any crimp.
Maybe you can get better combustion with these fast burning powders with a good crimp.
(All rules of trying unknown load recipes apply)

Powder reload 1:
Hercules Unique, 24grs
OAL 90,0mm
shoots 4" low at 50m with standard sight setting
Load can be increased in my rifle.

Powder reload 2:
Kemira N320, 30grs
OAL 90,0mm
some black residue on brass mouth after firing in cold weather
(not enough pressure to seal the chamber)

Powder reload 3:
Kemira N320, 35grs
OAL 92,0mm
at 50m: point of impact is dead on
brass is shiny

I estimate the velocity between 1300 fps and 1400fps (for 2550 - 2900 Joule).
recoil is easy, and no barrel leading.

best regards
Rolf