That double rifle thread is a interesting one, details some of the pitfalls and what can be done.
Thing to consider is the customs officer has broad authority, he can say it's ok stamp and you are out the door. Or he can refer to the commodity specialist for clarification. It makes a difference were the port of entry is as well as the ultimate destination. Customs is supposed to honor state regulations, if arriving New York going to New Jersey one thing. Container with firearms arriving in bond in Texas going to Texas quite another.
Boats
