Steve.... HE Grade Foxes (Super Foxes) with FACTORY 3-inch chambers will measure "short" at about 2-11/16 to 2-3/4". A special, tight chamber reamer was used to center the shell = a perfect fit and a consistent transition of the shot charge into the forcing cone and bore.
Earlier GRADED 12 gauge Foxes (let's say < serial number 27800 which was the first documented Super Fox) were available on special order with factory chambers for early 3-inch shells. Very few do have long chambers done at the factory. Those that do will measure a full 2-7/8" with a regular chamber gauge. I have never seen one that has a chamber length stamp. A letter from John Callahan, or a card check done on member request by the A H Fox Collectors Association, is the only way to verify factory originality. I do own such a gun, an early CE made in 1912, that has heavy 1-weight tubes.
In sum, a Super Fox/HE Grade 12 gauge 3-inch chamber made for progressive burning powder loads like the then new Super-X shells, will measure about 2-11/16 to 2-3/4" with a regular chamber gauge. And an earlier 3-inch chamber intended for pre-progressive (bulk or dense powder) loads in 3" long hulls, will measure about 2-7/8".
I hope this is helpful. Silvers
