I think...………...…. I THINK...………..the Model 21 's in 16 gauge are built on the 12 gauge frame.
I don't have my 12 gauge 21 at home at the moment. But the 16 with 28" barrels weighs in at just under 6 3/4 pounds. Still feels wonderful in the hands. But, seemingly, a good bit heavy, I'm thinking, were it on a smaller frame.
When the 12 gauge comes back home, I can give better info, I reckon
The 16 gauge 21s were built in a 16 gauge frame. I think these feel better than any of the 21s.
