OK a lot of comments about hooks etc. but if trying to fit a set of barrels of a different calibre have you considered the following -
the width and size of the lumps.
the distance between centres of a 12 and 20 bore gun.
the position of the loop .
will the 12 bore forend wood fit the 20 barrels with out looking stupid and will the connect up or are you planning separate forends.
The width across the face of the 12 as compared to those of a 20 as well as the height above the flats .
Probably a couple more things I've missed but those are main ones .
