Need help getting consistent cheek weld on my Remington 870 with slug barrel, scope, and tactical stock. When I'm using a laser to bore sight it, I get a different point of aim every time I raise my head and put it back down. It's only a little bit off, but how much will it effect actual location of the bullet? Don't really want to waste a lot of slugs only to find out the gun won't shoot consistently until I have a proper cheek weld every time.