Ted;
I was thinking the same thing about this gun may have been misidentified as to gauge. That 16.0mm would be .015" overbore from nominal for a 20ga but .032" under for a 16. It could of course go either way.
I seriously doubt that it's an 18 gauge, though even in my very meager shell collection I do have one 18ga shell.


Miller/TN
I Didn't Say Everything I Said, Yogi Berra