Thanks for the replies they gave me a thought. I'm now thinking it was where I rested the rifle, more central than near the bipod stud. I've just got a clamp to fit a tripod & realised this is the only rifle I've tried in it, normaly it's bipods or a simple large U shape on a tripod. I was shooting another rifle today on a bipod with the same scope & no problem. Packing a bipod next time I use this rifle & closely scrutinising how I use the clamp.