Your right, misinterpreted that part. But the forcing cone is defined as such :
Forcing cone - The tapered section at the front end of a shotgun chamber by which the diameter of the front end of the chamber is reduced to bore diameter. (
https://projects.nfstc.org/firearms/glossary.htm#F)
Therefore one should conclude that the bore diameter starts after the forcing cone.
Also the RCMP recognizes the measurements to classify a firearm as the nominal measurements, being under 20mm for both 10 and 12ga. Back bore or not, forcing cone or not, they recognize a 10ga shot as having a 19.69mm bore and a 12ga shotgun as having a 18.42mm bore.
The law should be revised accordingly so as to leave as little room for interpretation as possible, but I don’t believe anyone is at risk of being arrested by a police officer simply for having an otherwise legal shotgun in their possession (unless some other criteria were to make it so of course...)