It's unlikely you'll scratch the bores as good steel can be shot with steel jacketed solids (Rigby) from the turn of the century and later double rifles.
: Their light weight will reduce SD and BC, along with reducing penetration. They'll slow faster and will be more susceptable to wind drift.
: They would be more valuable for adding to a WW mix or with pure lead to increase the flow and help with filling out the mould. I wouldn't shoot them, but I also wouldn't be afraid of a modern steel barrel being harmed by them.