If the box is long enough and the right shape its obviously a good chance its a rifle. They might ask if its a firearm. If they do I would be obligated not to lie. If they know its a firearm their policy states that they can require that the firearm be checked to be sure it is unloaded. My postmaster has twice doen this to me and once it was a very tactical looking AK-74 that had everybody on edge, even me! Funny! Now my blood pressure shoots up if I have to walk in there but since I have a FFL-03 I fell a little bit better about it.