How in the world would you go about proving parts purchased pre-ban weren't assembled pre-ban? What if I have parts on the shelf today, they threaten to "ban" AR's next month, so I assemble them tomorrow, then take them apart and put them back together after the "ban"? This stuff is beyond ridiculous.
What If I have two pre-ban rifles that I built out of various different parts? Essentially two of the same thing – maybe one has a scope and an adjustable gas block, and the other has a dot and standard block. Maybe one has a 2-stage trigger and the other a single-stage, and they have different brands of stocks. How would anyone know/why would they care if I swapped the stocks, swapped the uppers, etc?