In reference to this
I know of a 20G "French" SxS at a LGS that I will look at next week. Ive got 65mm 16's on hand but a 65mm 20 gauge is a different situation and I would probably want to get it lengthened.
I only have end-of-chamber wall thickness for one French gun
1950s Verney Carron 12b., 2 5/8" chambers: 0.100"
For interest - 4 Francottes
Francotte (1938) 12b., 2 5/8" chambers: 0.098"
Francotte (1894-5) 12b., 2.5" chambers: 0.085"
Francotte (1896) 12b., 2.5" chambers: 0.085"
Francotte (1930) 20b., 2.5" chambers: 0.095"
Possibly those with experience with French guns could share the end-of-chamber numbers
