Not always, but its a good idea. If your going to use full house loads with a gas check, its not as critical as the bullet will "bump up" to the bore size. If these are for a revolver, the cylinder throat size is also important, as the bullet must pass through it first. I have a 41 mag that has a .411 throat and a .410 bore which should be perfect for .411 cast bullets, but they lead horrabily- go figure.
