If the lockup is tight and the headspace is within spec, the forend spacer is probably worn and a shim needs to be placed between the forend and the spacer, then reinstalling the spacer to the forend. I've been experimenting with aluminum tape that measures .004" thick, it works well to tighten the action when open and to tighten groups on some rifles, but too tight(too much shimming) causes groups to string.