The inside of the barrel goes from chamber diameter to bore diameter to choke. Between the forcing cone and the choke, the bore is just a cylinder. The outside diameter of the barrel may increase a bit towards the muzzle end to accomodate the choke tube.
True with most shotgun mfgs but the newer Fabarm shotguns don't adhere to this inside bore spec. They use a "Tri-Bore" type of ID bore.
