Ken Farrell rails & rings & don't look back.
op,
400 series magnetic stainless will rust/corrode,since it has basically more chrome that chrome moly 4130 steels.
300 series non-magnetic stainless will NOT rust/corrode,but is NOT heat treatable & basically has more nickel added.
I wanted to purchase a KF rail, but couldn't get anyone to answer the phone or return an email, so Badger sold me a rail.