The proper way to measure a SXS or O/U frame is the distance between the firing pin holes. A 20ga frame that can be barreled for 16ga barrels is to big and not really a true 20ga frame. Look at some prewar European guns for a real 16ga frame. In addition a 16 should weigh less than a 12, many do not.
For the most part that applies today. However, at one time companies like Parker made different frame sizes for the same gauge. There are rare examples of 10gauge barrels on 20gauge frames.
A 16 gauge on any 20gauge frame will be lighter with the16Gauge barrels than if it had 20’s.
