Under the National Firearms Act it is illegal for a private citizen to possess a sawed-off modern smokeless powder shotgun (a barrel length less than 18 in. or 46 cm and an overall length less than 26 inches) without a tax-stamped permit from the Burea of Alcohol, Tobacco, Firearms and Explosives, which requires a background check and a $200.00 fee for every transfer.

I am not aware of any companies that make a 18" barrel for the Mossberg 500, most tend to go with the 18.5" barrel just to be safe (I presume).

You can cut the barrel off if you want to, just make sure you have a straight cut....personally though I would just buy the 18" or a 20" barrel and add an extended mag to it.

Pattern some #1 or larger buckshot (remember you are responsible for every pellet that comes out of that thing).

Put a good flashlight on it, or a Surefire forearm, get some training and you are good to go.