It would be closer to a 1.125 gauge. The gauge of a weapon is determined by how many spheres of solid lead the diameter of the barrel it takes to weigh a pound. The bore of a 12 gauge is 0.729" or 18.5mm. Since the weight of a sphere varies by the cube of the diameter, a rough doubling of the diameter increases the mass of each sphere by a factor of (40mm/18.5mm)^^3 or 10.11 times. Divide 12 gauge by the factor of 10.11 to get the actual gauge.
You'll have to do that. I'm not worth a crap at math.
Check out my links in my post #35. (Or use the links on my profile page) I got 1.2ga from them, and if there are math errors it's my fault, but the mistakes were made by the sites.