Thanks to all for providing insights. I have what was believed to be an English 12ga, that 12ga will not chamber a 12ga plastic hull shell.

Thinking that perhaps its really a 16ga, I tried a shell. The rim of a 16ga shell will stop the shell from falling all the way into the barrel, and is very loose in the chamber. A 16ga chamber gauge will pass the forcing cones.

The gun is a William Ford Eclipse BLNE, made after 1875 from what I understand, and would like to figure out what it shoots.