Here is letter from CFC / RCMP:
Dear #########,
Thank you for your telephone call today with regard to the legal classification of the Remington 870 shotgun and the MCS system, ################################################xx ################################################xx ############. Short barreled pump action shotguns with folding stocks are controlled within the Criminal Code of Canada, and it specifically details that the firearm is reduced to a length of less than 660 mm by means of folding, telescoping or otherwise. The otherwise does not take into consideration replacement with commercially available stocks such as a pistol grip stocks. Thus it is very legal and does not change the class of a non semi automatic firearm if you put a manufactured short barrel and pistol grip on the gun.
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I trust this response has met your needs, please feel free to call if I can be of any further assistance.
Regards,
#########
Manager
Firearms Technical Analysis Section
Canada Firearms Centre | Centre des armes à feu Canada
Royal Canadian Mounted Police | Gendarmerie royale du Canada