In my 44 magnums I cast and size to .431 . In 45 Colt I size to .454 , 41 magnum .411 . The size of the cylinder throats and the size of the " groove " diameter of the barrel is very important .
One place that many don't consider is the junction where the barrel and frame come together . If the factory had to really " torque " the barrel to index the front sight , there will be a tight spot at that junction point . I had one revolver with a 4 thousandths "choke " there . That is huge . With a little effort and patience it was easily removed . Good Luck