There are other ways to deal with tight screws in a new gun. I'd put impact driver right up to last resort on the list. In my example above, the old screw was buggered beyond repair. I had tried penetrating oils, heat, the screwdriver with a ratchet and it still wouldn't break loose.
In the case of your gun, I would have tried to secured it in a vise the best I could without damaging it. And try using a screwdriver with a ratchet wrench. Better yet, with the bit in a drill press.