As is apparent from his post, the OP is aware of the fact that he not only has to concern himself with different states and the District, but cities such as NYC.
No offense to the corrections and other LEO's, but the laws of the various jurisdections are so confusing that I would be careful just asking any officer. Even the folks at our AG firearm division have trouble correctly explaining Cali firearm law. Still, if at all possible check them out on the equivalent of state attorney general web pages and don't forget the feds.
Lastly, "HIDING SPOT". That smells like a concealed weapon. Here in Cal, where an unloaded shotgun can be in the car, I don't believe I can toss a blanket over it or otherwise hide it.