I don't know what diameter bullets are used in the .500 BPE rifles. .006" deep grooves in a .488" bore would put you right at .500" for groove diameter.

But the groove diameter of my .450 BPE rifle is right at .458", and I'm going to try to alter a .452" caliber mold to drop a .452" diameter bullet of about 300 grains that I can paper patch to groove diameter.

EDIT: I see references to either .510" or .512" diameter bullets for .500 BPE. I'm stumped too.

A true sign of mental illness is any gun owner who would vote for an Anti-Gunner like Joe Biden.