What is to stop the ammo manufacturers from building plants in each state? Then interstate commerce does not apply.
The federal government maintains that in-state sales affect interstate commerce; the federal courts agree this is a proper exercise of power under the constitution.
The ammunition and firearms manufacturers can make as many factories as they like, and can limit (factory) sales to in-state. The feds would still claim the right to regulate the actions, and the feds would use all force necessary to enforce its edicts.
I also see the fruition of multiple gun plants/companies going up all over the place.