I would keep the steel, whether it be brush or wool, far away from the pistol's surfaces. Even working with an oiled surface, it's the steel that will leave more evidence of cleaning efforts. Bronze brushes or Chore Boy copper wool won't degrade the surface as steel brushes and steel wool will, however minutely that may be. Any copper residue that may be left from the bronze brushes or copper wool wipes away with good ol' Hoppe's No. 9 on a nylon brush or handy rag. Surfaces remain unscathed.