Not kidding. If it a 12 Ga. the bore is about .071 ish. If it is a 20 Ga then you are back to the original question. But if you put an 18 in bbl on it it is just a regular shotgun.
2) any type of weapon by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, the barrel or barrels of which have a bore of more than one-half inch in diameter, except a shotgun or shotgun shell which the Secretary finds is generally recognized as particularly suitable for sporting purposes