You may want to remove the barreled action and take a look at the inletting to be sure that there is not a bedding issue that is not letting the action and barrel settle in properly.
If the bedding looks ok then try tightening the action screw tight - around 25 - 35 inch lbs, and the barrel mounted dovetail screw just slightly snug. I had a heavy barrel 452 and that was its preference for shooting the best groups. I have never had as good accuracy out of the heavy barrel as the sporters and I think that it is because the tension on the barrel can be so sensitive.
drover
