Impact driver, NO!
Use a hollow ground straight bit that fits perfectly with the slot. You can dab a touch of valve grinding compound to the tip (available at any auto parts store).
Use 1/4" drive ratchet. With socket that fits the bit. (Typically 1/4")
With rifle in vise, use one hand on head of ratchet, the other to apply pressure.
Remember, righty tighty, lefty loosey.
