RE: Mossberg 500's
What type of sight do you have on your gun? If all you have is the front bead, your eye may be too low on the stock. Your eye acts as the rear sight on a shotgun and can cause you to shoot high or low. I've found that recoil makes me want to hold everything tighter and I sometimes get down too far on the stock myself. You may want to look into getting a scope or some adjustable sights.