Or, if the local cop thinks there is a law, and most do know there are laws about "sawed off" rifles & shotguns.
I for one, would not care to spend my weekend in lockup, waiting to see the judge on Monday, so he can throw the charges out.
Even if you are calm, reasonable and rational sounding, the cop isn't going to listen to YOU. Having documentation from a recognized law enforcement agency (such as ATF) that your gun is legal in all ways, MIGHT be enough to convince the officer. If so, fine, if not, it hurts nothing to try...
A friend of mine has an antique motorcycle (1940 Indian, sweet bike), under the law of our state, as a registered antique (which it is) he is not required to wear a helmet when he rides it. He carries a notarized copy of the law with him when he rides without a helmet. Says that gets him out of getting a ticket about HALF the time. The rest of the time, the cop says "you may be right, if you are, the judge will throw it out, here's your ticket, have a nice day.."
To date, every one of those tickets has been thrown out, but he still has to challenge them in court.
So if you've got a gun that is legal but could be mistaken for an illegal one, best to have proof WITH YOU about its legal status. Might not keep you from being booked, but if it does, its well worth having it.