I just picked up an original 20ga. SXS flinter on Tues. Barrel length can be all over the chart, depending on when made, where made and ga. This 20ga. has 28" barrels. I have another original in 18ga., that has 32" barrels. The 20ga. original that I sold to my Dad has 34" barrels. I did have a 16ga. built from Ithaca Flues barrels. They are Damascus and after Nock style breeching are 28". Yes, the ribs didn't look right and with careful setup, a ball endmill made them concave and proper looking. I did use L&R Manton locks. Because of that experience with L&R locks, again, I would do all I could to reconfigure the Siler/Chamber locks to make the gun. May not be that close to 100% correct, but it may solve the issues I have had with these locks. I had seen where I think it is Fort Hill, is making a precarve SxS stock. I have no idea how it is set up, but it is for a perc. gun, but possibly easily adapted to a flint gun.