I recently mounted a Bushnell .22 rimfire scope on my Marlin model 60. I *thought* I had tightened the screws down pretty tight. When I sighted in last week at 50 yards my best 5 shot group was barely over an inch across. Today I took it to another range and was shooting at a distance of only 25 yards, and my shots were all over the place. By the end of the 200 round session all were hitting far right and low of point of aim. When I got home I realized that somewhere along the line the scope had slipped back far enough that the rear mount was completely OFF the end of the mounting rail. Anyone have any idea how to keep this from happening again? The picture below shows the screws on the scope. I am wondering if I need a special screwdriver or driver bit for a drill to get these tightened enough? The widest screw driver bit I own still only made partial contact due to the big hole in the center of the head slot, so I tightened it as tight as I could without risking stripping the screw heads. Apparently it was not tight enough.