A national firearms registry, or any state registry to which the feds have access, is unconstitutional.
If you read the comments, you have to take away the fact that states instituting their own AWB with grandfathering requires the owners to register their AW. Without registration, the state doesn't know who owns what so they can confiscate unregistered AWs that were acquired post-ban.
Catch-22: Can't have a national registry -- can't enforce a grandfathered national ban without a registry.
So, when faced with this conundrum, what "solution" do you think Congress will choose? Remove the "grandfather clause" to make ALL EXISTING GUN OWNERS instant felons, or create an unconstitutional registry?
If they choose the latter, the registry would make Universal Background Checks also enforceable for private transfers.
If Congress is conscious, they may see how the prohibition on a national firearms registry blocks other measures they want to pass. I'm betting they will start pushing for a registry. Without it, any grandfathered gun ban or UBC is moot.