Yes, there are laws against what you are suggesting. You cannot manufacture prohibited devices in Canada without authorization. Technically, any here who has assembled an airsoft gun from scratch has broken that law. The occurance right now is so low compared to straight transfer of complete guns that it really isn't worth any notice by the authorities. While one would avoid dealing with the CBSA by manufacturing receivers in Canada, the RCMP isn't exactly on our side either. The cost associated is also quite high, with dubious odds of recovering said investment without some extraordinary effort (such as finding a market beyond the very limited Canadian one) and luck (not getting the police all over you).
Please use the search function to look into your idea, odds are that the idea has been thought of already.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."