Here's the law:
A short barrel is not a prohibited device. (which is why 14" factory barrels are fine on Rem 870's). A firearm with a shortened barrel is a prohibited firearm. (I checked the definition: a 3" handgun barrel IS a prohibited device)
Constructive possession is not American law. It is a legal theory commonly used in all sorts of Canadian prosecutions, including the DUI cases I mentioned.
I am not saying these parts are not legal to own in Canada. Marstar knows the laws well enough to not do something illegal. Obviously, bringing them to the US is a no-no.
My point is merely this: owning these parts COULD
conceivably become part of a larger issue you may become involved in. Is it likely? Probably not. For me, the benefit of owning an autosear doesn't outweigh the risk of me forgetting it in my parts box when I cross.
And if I am misinformed, please do cite the law or regulation. If I am wrong, I will admit it.
And for what it is worth, constructive possession is an issue for me: the SAFE Act in NY defines an assault weapon as a semi-auto having a detachable magazine, and
- a muzzle device OR threaded barrel for muzzle device OR
- a pistol grip/thumbhole stock/vertical foregrip OR
[a bayonet lug OR - a folding/collapsible stock
My idea for transporting a purchase OUT OF NY was to separate an AR into upper and lower halves, removing the buffer tube from the lower. This way, I figured, there's no issue: at no point do I have a rifle with those characteristics. A lawyer at my NYS club advised me against that: simply having all the pieces simultaneously makes it an assault weapon. Does it matter that I am exporting it? Nope.
Do what you like. Its a free country.
FF