Wow, I never took drill that seriously before. Just remember doing drill with 'arms' in your backyard may induce an ERT response, and severe embarrassment, and unwanted publicity (even for your Pres Unit).
The untrained general populus can't tell a replica from a a real one. Remember that! Plus from the police point of view, from a distance replicas can fool them too. Even pointing a replica firearm is a criminal offence.
As for deactivated firearms coming out of the USA, in the USA once a machine gun always a machine gun, deactivated or not. If its a real reciever, even if its demilled, it has a paper trail. However one can buy dummy receivers with some real weapons parts, but thats usually M60 or 1919A4 .30/M2 .50 dummies. Usually the bigger type wpns for re-enactor groups, and display purposes.
The majority of deactivated weapons coming into Canada are from clearing houses in England, where before they are exported, they have been rendered inoperable with proper certification.
Mate, just stick to doing drill with arms with your PRes unit, and leave it at that. Besides many including myself find your reasons for wanting such a weapon rather, well sort of wierd (not that I am calling you a nutter).
On the flip side to LEGALLY purchase a functional AR15 rifle, to hone and improve your marksmanship skills, utilising CF range style practices is okay, but then you are limited to 5rd mags, and being caught with a 30rd mag too is a criminal offence.
Obtaining a firearm, and looking after it IAW all rules and regs has a serious amount of responsibility which goes along with it. AR15s and the like are classed as 'restricted' and come under the same laws as handguns, etc when transporting for lawful use, and each province has their own way of doing things. Other words, more freedom the further west you go (to some degree anyways).
If I was you, just be a part time soldat, learn and excell at the same speed of everyone else, and listen to your instructors/senior leaders to get it right. personally, I would 'shytecan' the whole idea.
Cheers,
Wes