What if I don't have an “AR-15 but I do have stripped lower, an upper, and a lower part's kit? Do I have to register that non-AR-15?
How about a less than 80% lower, plus jigs, an upper, and a lower part's kit? Do I have to register that?
What if I have a legally purchased AR-15 lower that went through an FBI instant background check and an extra upper burred in a closet somewhere? Does that require registration as an AR-15?
Can you see that this could get complicated because you first of all don't know what you are trying to legislate and no one in their right mind will help you.
They will want you to register a block of aluminum next.
In CA, the “assault weapon” registration required a fixed configuration. The registration required submission of pictures showing the assembled upper and lower, barrel and receiver stamps, etc. As far as compliance, it has been an utter failure, with few people willing to place their weapons on a confiscation list. As far as I know, pretty much every state that has passed registration has seen very few actual registrations.