I can address the issue of 20 ga. guns, but will have to leave the 28s and .410s to someone else.
As to the twenties, it's kind of a moving target. In broad-brush terms they don't require as much constriction. A 12 ga. gun will have 30-40 mils of constriction for full choke, but I've rarely seen more than 25 mils on a full choke 20 ga. Then there are the exceptions. I have a few 20 ga. barrels in the 15-20 point range that throw legitimate improved cylinder patterns. And is it my imagination of did Winchester seem to get away with less constriction than other manufacturers?
