Chrome lined brazed together barrels along with at least 1200kg or 1200bar proof are preferred. The only Spanish guns made for steel use in larger numbers were O/U guns by Laurona or Lanber (don't remember which). These had special rust resistant exterior metal finish and steel shot proof that is marked with symbol similar in appearance to one used by Boy Scouts. I don't see how upland steel loads of 6s, 7s, or 8s would be that bad but I would check with Kyrie on Spanish gun forums of Shotgun World. They do have rules of Spanish guns posted there.