iANAL, but I would imagine local laws are applied locally.
For example, I can buy an AR-15 without a pistol grip in California, travel to any free state and attach a pistol grip to that rifle. Doing that I would not violate California law, even though AR-15 with pistol grip is illegal in CA.
For example, I can buy an AR-15 without a pistol grip in California, travel to any free state and attach a pistol grip to that rifle. Doing that I would not violate California law, even though AR-15 with pistol grip is illegal in CA.