Remove the cylinder and put the barrel in a good vice with padded jaws muzzle down. I pad my vice jaws with thick wool inserts from Sorel boot paks and glue them to polycarbonate backing plates . Crank it down good and snug,drop a ball or bullet into the barrel forcing cone. Then find a short piece of hard wood board, place into the cylinder window on top the ball and strike along side the frame with a dead blow hammer on the board opposite the side your hand is on. A bullet will be better because a ball may not be big enough around to get into the barrel for a good land engravement.
Put a wood dowel in the muzzle and drive the bullet back out.
Now mount the revolver horizontally in the padded jaws and drive one into the muzzle. Carefully center a sheet rock screw in the middle , turn in a good ways and pull the ball out by clamping the screw head in the bare vice jaws and pulling the revolver away from it. These pull screws left in the ball/bullet are good handles when measuring. Mike D.