2. Shell length refers to the length of the shell after it has been shot; with the crimp fully open. An unused 2 3/4" shell will usually drop into a 2 1/2" chamber.
3. The 1907 Hunter Arms chamber engineering drawing shows the 12g chamber is 2 3/4" and tapers from .811" to .797" at the entrance of the forcing cone. If you are using a rule measure, it might we wider than .797" and stopping short of the end of the chamber.
4. Can you measure the diameter of the Rio shell and the end of your chamber rule?