Gunrunner hit the nail on the head. Assuming its mounted to stay put in some way, the issue is almost always one of it staying at zero - that is, the windage and elevator adjustments shoot loose or on some junk I've seen, could loosen up and change the POI just by shaking it. +1 on the boresighting - I've saved a fortune on ammo since I started with an in-chamber laser. Just did one the other day on a 556 build with a 3x9 Leopold. Chambered the laser, turned the scope dials to line up the crosshairs, 1 st 3 round group was right on the paper at 100y. A coup,e of clicks and done.