The feds can assert jurisdiction by virtue of the interstate commerce clause. Most firearms and most ammunition pass over multiple state lines before being sold, which allows the feds to preempt legislation based on the fact that the items are in interstate commerce. As to national reciprocity, there are multiple prongs on which the feds can assert jurisdiction. First, of course, is the Second Amendment to the federal Constitution. Then there's the fact that the guns and the ammo are in interstate commerce. And then there's the fact that when a person travels to another state, he or she almost certainly spends some money there so the person is also engaging in interstate commerce.
If the feds can argue (as they have) that they have jurisdiction over marijuana that was grown, sold, and smoked entirely within one state on the grounds that by growing, selling, and smoking that weed the pricipals affected interstate commerce by NOT importing the marijuana from another state, they can certainly assert jurisdiction over things that really ARE in interstate commerce.
It would be great if such legislation could get passed that would outlaw the state bans, but I think it would be struck down by the lefty courts.