The point of what I wrote earlier about not measuring around the fences was more about the overall size of the action - there are 12 bores built on 20 gauge frames, ans "1620s" are relatively common.

Toss in round bodies, lightweight English 12s, and actions like the Ruger Gold Label, and while the measurement around the fences will be within a predicable range (two 12 bore chambers and the sides of those chambers) we'll get a much better idea of the mass of the action by measuring right behind the fences.

There's lots of other variables, but it's a better starting place than measuring around the fences.