We've been here before just recently, but I can't find the link. Maybe someone else has it to help BB308 out?

The reason this is a fairly pointless exercise is that the barrels on a SxS double rifle are not straight. They start out parallel, and perpendicular to the breech-face at the chambers, and are curved inwards somewhere towards the muzzles to achieve regulation. The spacing and amount can vary substantially between manufacturers and indeed among their individual offerings.