I had an 870 Marine Magnum. We used to go fishing in the back country in B.C. and I used to pack that just in case a grizzly bear came upon us as we were fishing. It's obvious that it is included based on the definition, but the concern for many firearms owners across Canada is the variant option.
As the basis of the gun or the firearm is an 870, a well-known firearm that's probably been in most farmers' gun vaults over the last 100 years, the concern is that while this one might not be included, what about all the variants? That would put many of the shotguns under this prohibition.