For best results, resize brass before using this tool. Reason: So case mouth inside diameter is consistent. Loosen thumb screw; slide cutter in or out to the desired location; securely lock screw. Chamfer should be approximately 1/32" (.031"). Cutter is sharp and will remove material until case mouth hits depth stop. Go gently; don't remove too much. Bullet will now seat smoothly with its base unharmed.