I have had good success by installing a soft neoprene O-ring between the stock and the barrel about 1/3 back from end of the stock. The soft neoprene conforms to the contour interface between the barrel and the stock, providing support and absorbing barrel flex. This works about 90% of the time. It doesn't work as well with rubber O-rings, as they are less pliable than neoprene.
