Reasonable restrictions also might be thought consistent with a well regulated Militia. The registration of firearms gives the government information as to how many people would be armed for militia service if called up. Reasonable firearm proficiency testing would both promote public safety and produce better candidates for military service.
If they want people eligable for the militia to submit their names and contact info, that is one thing. They don't need an inventory of your equipment.
Also, they may want proficiency testing for membership in the "organized" militia, but not as a condition to own "arms".