This would merely be the most extreme example of nullification. And it HAS happened historically, and even currently. Recently, Wyoming passed one of those "firearms freedom" laws and announced their intention to arrest any federal agent who attempted to enforce federal firearms regulations within Wyoming. Feds haven't tried their luck yet. Would be interesting to see who blinked if they did.
Until it reaches that extreme, it is not unconstitutional.
I hadn't heard of the Wyoming case, but if the details are as you say, it would probably come down to whether the federal firearms regulations at question are themselves constitutional or not, as decided by a court. If it is deemed to be so, my guess is Wyoming is out of luck should the feds ever try to go in.
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