A Lot of this depends on what one refers to as "Frame Size". Plans & Specifications book for the Smith show the bar & action body of the FW's as being the same for each gauge. Width, Height & firing pin spacing of the standing breech vary per gauge.

Question is does this constitute a different frame size. In some folks eyes it does, in others it does not. Parkers were pretty much the same, all their vaunted Frame sizes consisted for the most part only differences in the standing breech size.

For barrel interchangeability of course they have to have been built to essentially the same breech size. Even guns of the same gauge were built with different sizes of breech according to barrel weight. I have three different sizes of breech, including firing pin spacing's, in 12 gauge only among my Lefevers, but the rest of the frames are the same within manufacturing tolerances. A 16 gaugre with XX marking has a slightly narrower bar, while a 10 lb 10 gauge has a slightly wider bar. Breech sizes also vary on these as well.

Thiough I don't have any to measure I would highly suspect that the fultons noly vary in the breech size, but that there would be enough difference in this area between a 20 & a .410 to prevent interchange.


Miller/TN
I Didn't Say Everything I Said, Yogi Berra