Such an act could not trally be enforced retroactively, unless people volunteered to register the guns they already have. Criminals in illegal possession of guns cannot even be compelled to comply with a registration program (USSC, Haynes vs. US, 1968) because it would involve self-incrimination, so maybe that would also provide an affirmative defense to anyone who simply refuses to register their existing collections.
Sadly, law-abiding citizens would enjoy no protection from forced registration under Haynes
. Criminals who illegally possess firearms cannot be forced to register them, because registering them would force them to admit they possessed the guns, thereby incriminating themselves.
For regular citizens who possess their guns legally, forced registration doesn't lead to self-incrimination, so Haynes