Lead conicals will lead your bore if you try to push them too fast. 1200 - 1300 fps seems to be the limit, a lubed wonder wad can sometimes get you a bit more speed.
Think of it this way... have you ever leaded the bore of your .22lr? Most of the ammo you shoot is most likely a pure lead bullet. Go to the super fast stuff and it will be copper plated.
Same rule applies here...