I posted this on another thread concerning this same problem. The original threading will strip with even 15 in/lbs so DON'T use a torque driver, just snug them. BTW, I learned the hard way and found out that three screws are not good enough for high recoil (had it mounted on a S&W.44 mag and the sight ripped right off the revolver). I also thought the sight was junked but refused to give in without a fight. For reference, as below, rather than selling the sight for nothing.
"FIXED IT ! Use a 7/64 drill bit, mounted in a t-handle, to clean out the remaining thread damage. Get a weaver rail for a Bennelli Nova, complete with the screws, and toss the rail. The screws will tighten in perfectly if you use straight, even force, no stripping and no protruding screw head out of the mounting plate countersinks. I thought this sight was done for so nothing ventured, nothing gained. Needless to say, the mount won't be coming off again."