IIRC, gauge is a function of bore diameter and weight of shot. 20 gauge means that it takes 20 lead balls that diameter to weigh 1 pound. Ditto 16 balls to the pound for 16 gauge, 12 balls to the pound for 12 gauge. So in a theoretical 1 gauge shotgun, the bore would be such that 1 lead ball would weigh 1 pound. So a .12 gauge would be, as somebody has already stated, the size of a field artillery piece or maybe a 5 inch dck gun.
Or, on the other hand, I guess the .410 gauge measurement standard could be used, .410" being the actual bore of the shotgun. In that case, a .12 would be about 2/3 the size of a BB (.177 cal.).
Now I'm gonna pour a shot of Chivas and ponder what I just wrote...
As I recall, one "gauge" is equivalent to the diameter of a one lb. ball of lead, and the higher the "gauge", the lesser the amount of the lead, and accordingly the less the diameter of the ball of lead used to make the "gauge", and therefore the smaller the bore of the weapon. Which is why a 10-gauge shotgun is much more powerful than a 12-gauge, which is much more powerful than a 20-gauge, and so on. So, by that logic, a ".12" gauge shotgun would correspond to a much greater amount of lead in the imaginary ball of lead (8.5 times more massive weapon than even the already-huge hypothetical "one gauge" shotgun--8.5 x .12 = 1.0), and therefor a much larger weapon bore diameter. Vastly larger than exists in the non-military world. Accordingly, the idiot who wrote the article was even more uninformed about guns than it first appeared.
It seemed a lot more humorous before I had to explain it in such detail.....