Watch out for this BS! One of the ways they get around states rights is to sign a treaty and the senate ratifies it.
Look back at hunting regs and it used to be each state would for example set a season for goose. The US & Canada set up the migratory birds act and viola! It becomes a federal offense to shoot a goose outside the prohibited parameters.
If the senate were to ratify a UN global gun whatever, it would take a supreme court ruling or a constitutional amendment to undo it.