I have fired a lot of factory steel shotshells through "open bored" older American SxS's with zero problems. Using both CYLINDER and IMPROVED CYLINDER steel barrels on my Remington 1900, A.H. Fox, and a Lefever. I have not used steel shot larger than #4. I avoid magnum loadings.

When I'm going to use tighter-choked SxS's, I handload NICE shot. It is pricey, but works well.

JERRY