The RCMP specifically measures barrels without removable elements installed so that they're consistent for legal purposes. Nothing about that is extraordinary, on the contrary, that is their standard operating procedure for legally assessing a firearm. I have serious doubts that they or the courts would care how SAAMI determines said measurement for technical purposes.
I'm not sure anyone seriously thinks that their shotguns will be seized in the near future. The issue is that as the regulation is actually written it can be enforced that way. What happens in two years after we've swallowed the idea of this and enforcement starts? Depending on what sort of buyback provisions are or aren't passed, you might start getting some takers...