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.
Not much has changed in this regard. If you measure chokes, which have the same designations but in differing guns, you will find that they are all over the place in actual constrictions, especially so as you get into the little guns. Even published choke charts, which supposedly give us the actual constrictions for the differing designations, have quite a bit of variation.
This is one of the reasons, and a significant one in my opinion, that some makes of guns got the reputation of "shooting harder" than others.
SRH
