The normal length for American 16-gauge shells at the time your gun was built was 2 9/16 inch. When the Super-X 16-gauge load came out in late 1922 it was a 3 dram equiv load containing 1 1/8 ounces of shot, still in a 2 9/16 inch case. Just before WW-II interrupted things the Olins added the 16-gauge Super-X or Super-Speed in a 2 3/4 inch case with a 3 1/4 dram equiv load with 1 1/8 ounces of shot. Remington brought this load out in the early 1930s for their Model 11 and Sportsman 16-gauge autoloaders, which were made for 2 ¾ inch shells. Between the 1954 and 1955 catalogues the Olins introduced the 16-gauge 2 3/4 inch Magnum with 1 1/4 ounces of shot. Powder load was just listed as Maximum.
Chamber depth is considered to be the joint between the chamber body itself and the forcing cone, which reduces down to bore diameter. This is loosely based on the length of the "Fired" shell. Today, theoretically when the crimp opens on the shell being fired the end would land at the junction of the chamber and cone. Prior to WW-II many companies had the practice of holding the chamber about 1/8 inch shorter then the shell for which it was intended. Fly in the ointment is nominal measurements often differ from actual ones in both chambers and shells. A very good method I have found of measuring chambers without much outlay of cash, and is quite accurate enough for virtually any situation, is a common 6" flexible machinist's scale which you likely have. Hold the barrels with muzzles toward a light source, not necessarily a concentrated one a window is great, while looking into the breech and the cone will be thrown in a shadow. May have to move the barrels around a little until it is distinct. While still looking into the chamber simply slide the scale in until you observe the end coming flush with the shadow line & mark position of breech end with your thumb. Remove and read the scale. I usually repeat this a few times to insure I am getting a consistent reading, but you will be amazed how accurate this can be done. While I own a Galazan chamber gauge, I use this more often than not. The chamber body itself has a taper of about .005" per inch. Sometimes chambers were cut with slightly worn reamers giving a slight undersize chamber. As you can readily see if the chamber should be .001" under gauge size at the end this would give a false length reading of about .200" with a gauge. The machinist’s scale method can be more accurate.