If they'll run at that speed....
I'd want to know how the bullet fits the throat, how hard they are and how well the lube works.
Data for the Rigbone is light, pressure wise, so the starting load of 22 isn't out of line.
Be sure you aren't getting grey leading at the muzzle after sustained fire and the bullets are jammed;
Bottom line, if you see no leading, no lead star on the crown and you get sustained accuracy as the gun warms up, you're GTG.
Run an LBT 375 GC from my Rigby often; its a great cast cartridge...
