Originally Posted by Goatwiskers
Shortest legal barrel is 18".
Not altogether true - the shortest legal shotgun/smoothbore length barrel is 18' IN THE U.S.A.
Other countries = other laws - the OP's from Canada, & IDK the law there.
Rustle In The Bushes: Mossberg 500 barrels are interchangeable, and sold in bird/buck combos - a 28" VR bird bbl with a 20" or 24" bbl w/rifle sights.
There's no reason you couldn't set up such a combo with the bbl length(s) of your choice, provided the work & lengths are legal where you reside.