Shotgun Barrel Length?

So, am I correct that Federal Law states that a shotgun must have no less than an 18" barrel? If so, can a person saw off the barrel of his own shotgun to 18.5"? Where does the chamber start and the barrel begin? I'm asking because I have an old 20ga over/under sitting in my old room at my folk's house with an inch of dust on it. I'm thinking about sawing down the barrel and stock and making it a home security weapon for my wife.

 

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