I would suggest you both go shoot the hell out of it at the range and see what it does. With it returning to zero all the time I would not think it was the scope but I am not an expert. Whenever I was a marksmanship instructor in the Marine Corps I would have guys shoot great all day one day and the next day not hit anything at all, it was never the rifle, always the shooter. 99% of the people I see go through the course and shoot poorly said it was the rifle but I would bet 98.9% of those were shooter error. There could have been a dozen other factors contributing to a miss with a rifle propped on a widow and him sitting on your knee also. Handgrip, trigger pull, muscle control, breath control, support (even if it was sitting on a window ledge), natural point of aim, eye reliefe, sight alignment sight picture just to name a few, try putting it in a vise, that is the only way to realy tell if the mount is steady. I know that I used to have a bipod on my 22-250 and it would shoot different freehanded than it would on bipods. A good day at the range with both of you shooting the rifle a lot should work out if it is the scope or not, then if it is I would trash it and buy a new one unless it is warranteed. I am not trying to insult your shooting ability or anything, just trying to maybe give some help.