10 GA??

I droped a 12 ga shell in the chamber and it seemed a bit lose but not a whole lot more so than on new 12 ga I have. I wondered about the 11 you mention myself but, as I freely admit, I am ignorant of what the markings mean.

So what is it really?

And are those Damascus barrels? I don't see a nice patern but then it is old and not in the best shape.