I figured that something was wrong with the bedding, nope, fired my accuracy load and it grouped tight again,...
Think barrel whip and the vibration modes (and nodes) that each particular load generates. When you visuaize things this way you realize that one vibration mode may have a null at a potential stock/bedding interferance point, while the other load may generate max
deflection at that same point. Bingo, ...you then have stock interaction.
Multiple discussions and visuals here: