I have wondered about this myself.
With Cap & Ball or Metalic Cartridge Black Powder Revolvers -
How was 'Proofing' done in the Black Powder era?
How is it done now?
With Long Arms, a double charge or double Projectile would be the recourse I think...but, a Revolver has only enough room the the Cylinder Bores for whatever the standard Charge it is supposed to use...so...