The exact amount of constriction has always varied some by maker.
That's a good point. Belgian Brownings are one example of guns that, generally speaking, have more constriction for a given choke designation than did other contemporary makers. A chart in Jack O'Connor's old classic "The Shotgun Book" shows that Browning's 12ga mod constriction was .024, compared to .018 for Remington and .016 for Winchester. That's a pretty fair spread. And if you put your bore and choke gauge into a Superposed or A-5, you'll find it has more constriction than what you'd expect, given the choke marking.
