Shotgun Barrel Length: Legality, Measurement & Preservation

Question
I have an old shotgun that my father gave me. The barrel is 29 inches from the tip to where the ammo is loaded.  THe top 6 inches is rusted and I have to cut it. Is that a legal length?... 23 inches.?  Is that how they measure.?

Answer
18 inches is the minimuim legal length.
Its measured from the begining of the chamber to end of the barrel. Cutting the barrel will remove the choke.
If its collectable gun it will destroy its value.
There are ways to remove rust without cutting it off.

I would also consult BATF before cutting any barrel less than 21 inches long.