The receivers of most SxS shotguns were color hardened. With age & use the colors mostly wore off leaving a somewhat shiny appearance. Only in the case of extreme neglect did rust take over, thus most "Antiquing" methods are not really appropriate for a SxS receiver. For bbls I once did a set using Laurel forge brownimg solution. I applied several coats & carded them without boiling as for a brown finish. Rust adhered very tightly & was difficult to do a good carding job so finish was a little "Fuzzy". After a good build-up was obtained I threw the last rusting in the tank for a boiling water treatment. Result was a sort of muddy blue which looked about like a 75-100 yr old set of blued bbls, turned brown. Most likely this would give similar results on a receiver if that is the type of finish you are seeking.
