A question much like this came up some several months ago. The question was from me.
I had looked at a gun that had 16 gauge chambers and 20 gauge barrels. Walls were right as were the bore sizes, forcing cones and chamber length was correct for the period. As I remember it, the gun did not seem to have been futzed with but I was leery of it and did not buy.
We never came up with a reasonable explanation for it, though.
Best,
John

Humble member of the League of Extraodinary Gentlemen (LEG). Joined 14 March, 2006. Member #1.