It will not hurt if you drill into your barrel "a bit". Determine how much of the screw will be threaded into the barrel and drill the holes slightly deeper than that. You may want to have two tap drills on hand so that you can grind one flat on the end to make a flat bottom hole. And you do not need carbide for a Mosin - all that I have drilled were relatively soft and drilled fine with high speed steel drill bits. Drill with the pointed drill first and then finish with the flat ended one. You will also need a bottoming tap to get the threads close to the bottom of the flat bottomed hole. This minimizes the depth the hole needs to be which should only be deep enough to accept the screw and hold the mount securely. Good luck.