Not that straight forward
and they said
"
was an almost linear relationship between bore size and pattern"
The size of a shotgun's bore has nothing to do with the overall size of its pattern. Air resistance opens patterns, and pellets react to air resistance the same way. Comparing larger and smaller bores is apples to oranges because of the difference in payloads.
No distinction can be made between a .75 oz. 1200 fps load producing a 65% pattern at 40 yards whether it came out of a 28, 20, 16, or 12 gauge. There is no tangible difference.
Also shot type/ quality, hardness and wad type used all comes into play. If you reload and shoot a lot of paper over time you can improve your patterns in any gauge. I have always found lower pressure loads always pattern better than higher pressure
LOL on the magic they said "
Yet in the field, the 28 does seem magical "
Take care