Thomas, In looking what makes the Longrange Wildfowl special whether it be over bored or longer choke tapers is what Hunter Arms did to make their guns shoot to where you wanted them to. If you ordered a gun and asked them to put 90% # 7 1/2 pellets in a 30" circle at 40 yards or 90% #4 pellets at 60 yards, I'm sure that they would do this and have done so. As far as SAAMI. specifications are concerned, a lot of these guns were built and chambered before 1926 and I'm sure that if you measure any shell they will not measure 2 3/4" or 3" when opened.
As far as guns that the chambers were lengthened, then we can't go by them. I still don't know why anyone would want to lengthen the chambers on an old gun, there are so many different lengths to go by, and how many that are lengthened have the forcing cone done, probably not many.
For the Wildfowl gun you are referring to, does it have stamped on the barrels Chambers 3" Inches, very possible it is 2 3/4" chambers.
I am going to get some brass and make up a few different bore gages at a friends business and will post what my findings are on the Longrange, either it's over bored or the chokes are longer. The only way to do it is through the breech and since both of mine have 32" barrels the gage should stop when I get to the taper and I can measure where it stops at.