A ruff bore will still shoot fairly accurate. The problem is, the ruff bore causes the fowling to become more of an issue. I was about to give up on a rusty bore, when I decided to try some powder called Black MZ. This powder leaves very little residue. Now the rifle shoots as well as another rifle, that I have, that has a perfect bore. The bore is pitted, and looks bad, but the rust is gone, and it shoots just fine with the Black MZ powder, and that saved me the cost of rebore.