Far as I have seen, the "common" Black Powder load for the .500 3" was a 440 gr. bullet; mine is regulated with "120 gr. of #6 Powder" as indicated on the buttplate. (Stephen Grant, JUL, back action locks)
The Kynoch Nitro for Black normally comes with a 380 gr.
That is if my memory serves me right.I dont have any of my notes here in country.

There are some other guys out there with all the detials, I may be worng on par tof this.

Brian
LTC, USA Ret.
NRA Patron Member
AHFGCA Life Member
USPSA Life Member