I start out by bore sighting my shotgun. I mount my red dot on the barrel, then I take the barrel off of the shotgun. I place a target at 20 yards, lock my barrel in a vise and look through the barrel and line the bullseye up in the center of the bore. Once I've done that, I adjust my red dot to the bullseye as well, then put the barrel back on the gun and shoot it. Just my way of doing it.