My nickel steel Springfield has a fragment of steel ironed into the bore surface about three inches back from the muzzle. I am reasonably sure it is steel, since electrolytic cleaning didn't touch it and fire lapping did almost nothing to it. Does anybody have any advice about trying to counterbore back far enough to remove it? I know I would need a piloted drill or reamer to have any hope of accuracy. What size bit would be advisable?