What brand bipod is it ? Are you shooting prone or off a bench ? Are You preloading the bipod ? Are You running a rear bag ?

The Harris swivel bipod con be adjusted for tension and better yet have a Podloc installed and be totally locked down stocks .

If you are getting good groups at 50 with the same technique this more than likely shooter induced and a position issue .

Come out to one of the long range matches at PWSA or CRC and someone can help get it sorted out between everybody there there's probably a century's worth of experience on the line .

The HS stock on that gun is plenty stiff enough and more than likely forend flex is not the issue . I know plenty of guy's winning the long range tactical matches with HS