My old French Made MAS SxS 16 gauge shoots brenneke slugs very well with both barrels hitting the same point at 55 yards, beyond 55 yards is where the barrels no longer shoot to the same point of aim, the improved barrel will still shoot to point of aim to about 70 yards, the modified barrel is toast after 55. The SxS with rib in the middle actually makes for a really good sighting plane.
good luck and regards,
graycg