Have the 20 ga Field. When I shoulder the gun my eyes r above the rib. Don't have this problem with the 12 Field Classic. Can I fix this problem? Higher rib? Shims btwn receiver & stock? The 20 came with s 24" fully rifled slug bbl & I'd think with a scope it'd fit fine. Would like to get things right with the field bbl rather than buy a 2nd gun. Any suggestions? I've read where there used to be shims you could add btwn the stock & rcvr, but I think they've been discontinued.