It all comes down to $$$. It’s perfectly legal to resell a long gun a million times with no registration, so they are basically impossible to track. Unless there was going to be a lot of money put into some effort to find and track what I would guess is over 10 million weapons it’s a law that is basically impossible to enforce. Also the lower reciever is the firearms so you could strip every part off of any lower registered to you and just turn in the lower and they can’t do shit. Then all you need is a different lower and to reassemble the rifle.