So you include the chamber on a shotgun then? What about with a rifle, you close the bolt and then include the chamber on that measurement too?
I believe on everything except a revolver the correct procedure is to measure from the end of the muzzle to the bolt face on a closed action. Usually done by inserting a cleaning rod to the bolt face, marking the position of muzzle on it, and measuring that length.
(2) For the purposes of this Part, the length of a barrel of a firearm is
(a) in the case of a revolver, the distance from the muzzle of the barrel to the breach end immediately in front of the cylinder, and
(b) in any other case, the distance from the muzzle of the barrel to and including the chamber,
but does not include the length of any component, part or accessory including any component, part or accessory designed or intended to suppress the muzzle flash or reduce recoil.