I don't believe the rifling, or lack of it, has anything to do with it being registered or not.
The CFC found a cartridge that had a muzzle velocity of over 500 fps (a less than lethal round which produced 550 fps) and as such, declared the 40mm launchers to be a firearm. And this was despite the fact you couldn't buy these rounds in Canada. This was around the time that Dlask's M203s were being seized, and certain departments were also trying to control the 40mm Milkor launcher that found it's way into private hands.
Around the same time, they also declared the RPG-7 a firearm, since it could be fitted with a subcal kit which would fire a round.
I have no doubt that when they have a reason or desire to control the 37mm launchers, they will either find a round or an adapter which will make the 37mm launchers meet the definition of a firearm.
Shame these departments don't have any criminal control work to do, and have to worry about collector's toy control.