Would a 12-gauge chamber that actually measures 2 1/2-inch be intended for 2 5/8-inch shells? Our U.S. ammunition manufacturers did not regularly offer a 12-gauge 2 1/2-inch paper shell. From the 1890s to WW-II our U.S. ammunition manufacturers offered 12-gauge paper shells in 2 5/8-, 2 3/4-, 2 7/8- and 3-inch lengths. As I mentioned above, from 1907 onwards all 1 1/4-ounce loads were only offered in 2 3/4-inch or longer cases and 1 1/8-ounce loads marketed as "Trap Loads" were also in 2 3/4-inch cases --
RANGER 12-gauge Super Trap 02.jpg
For a while before WW-I there were also 3 1/4-inch 12-gauge cases offered for bulk smokeless powder. I've yet to see a box of such loaded shells, but have seen these NPEs --
1901 PIGEON 12-gauge, 3 1-4 inch 01.jpg
1901 PIGEON 12-gauge, 3 1-4 inch 02.jpg
1901 PIGEON 12-gauge, 3 1-4 inch, NPE.jpg