Excuse the huge post. What I think you are interested in are the definition of "prohibited devices", while the authority to define these things comes from the criminal code, the Governor-in-council lists the devices in regulations. I'm posting the most relevant provisions that were formerely under the "Cartrige Magazine Control Regulations". If it does not fall under this list (be sure to check the regulation for the complete list) it is not a prohibited device. This is the same list that allows M-1 rifles to keep their 8 round capacity.
Home > Federal > Statutes and Regulations > Consolidated Regulations of Canada > S.O.R./98-462 > Former Cartridge Magazine Control Regulations
Regulations Prescribing Certain Firearms and other Weapons, Components and Parts of Weapons, Accessories, Cartridge Magazines, Ammunition and Projectiles as Prohibited or Restricted
SCHEDULE (Sections 2 to 6)
PART 4 PROHIBITED DEVICES
Former Cartridge Magazine Control Regulations
3. (1) Any cartridge magazine
(a) that is capable of containing more than five cartridges of the type for which the magazine was originally designed and that is designed or manufactured for use in
(i) a semi-automatic handgun that is not commonly available in Canada,
(ii) a semi-automatic firearm other than a semi-automatic handgun,
(iii) an automatic firearm whether or not it has been altered to discharge only one projectile with one pressure of the trigger,
...
