The barrels, usually 12 gauge, but, possibly 10 gauge, in this case, are machined to accept inserts in 16 gauge. They are permenantly installed in the barrels. There can be any number of reasons for doing this, but, you now have a 16 gauge chamber firing into 12 gauge barrels. I’m told they pattern just fine, and there is a bit of a pressure drop in the barrels themselves.

Any number of reasons, but, to me, I’d just look about for a proper 16 gauge gun.

Best,
Ted