Im guessing it would be a possible for an experienced gunsmith. I just had a set of 410 and 28 ejector barrels barrels fit to my 20 gauge by Bill Schwarz.
Of course the 20,28,and 410 all used the same frame. I would think you could fit the 16’s to a 20 gauge as you could strike the barrels to contour the bolsters.
Plenty of other guns have barrels fit up after deivery to make 2 gauge sets. Good possibility Winchester 21 is the same.
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