The G2 Contender pistol comes in a .45 long Colt and .410 caliber shotgun. Is there a 12 gauge shotgun barrel (12- 16") to fit the G2?

The rifled 45/410 is a special sort of rulebeater that has been giving the BATFE migraines for decades. The Contender G2 is not large enough to accomodate anything larger that 28 gauge and smoothbores must abide by the usual regulations.

You are restricted....as was stated by nord to 28 ga. max. The barrel diameter required to fit in the frame won't allow for a larger bore. Even a 28 ga. requires the foreend nuts to be soldered on the barrel.

Say I had a design that would work... What then? What would be the legal restrictions? Any?

If you have a design for a 12 ga. barrel that would work on a G2 frame I am sure you would have many interested parties.
With that said and based on the years of legal wrangling that has went on with the feds and TC over pistol shotgun barrels...not likely that they would except it with open arms. Not only a can of worms...a bigger one than previously opened.

Arctic Gun, if you are a Canadian, Russian, or Skandahoovian I have no idea what your regulators require. But if you are a Yankee then the Revenuers insist your smoothbore cartridge shotgun be at least 26 inches long and have a barrel no shorter than 18 inches (unless you wish to complete endless forms and pay special fees). Wearing only a pistol grip a Contender's barrel would have to be maybe 23 inches long for the entire gun to measure the required 26 inches.

Thanks Nord. That's what I wanted to know. I'm pretty sure there are similar rules here in Canada.