If you look at the full choke constriction as a percentage of the cross-sectional area of the unconstricted bore, you find that the "amount of constriction necessary to make that gauge deliver the desired choke pattern" is the same for the 12 and 20ga.
In other words, the area of a .695" muzzle is 92% of the area of the .729" cylinder bore. The area of the .592" 20ga full choke muzzle is 92% of the area of the .617" cylinder bore. You could think of full choke as an 8% reduction in bore area. From that perspective, the 12 and 20ga constrictions are actually identical....is that coincidence?
Maybe, but it's true for the 28ga as well. Using .550 for the bore and 22 points for a full choke gives 93%. Use 23 points and it comes out to 92%.
