This can be problematic with a shotgun because they don't usually have a rear sight. You learn to place your cheek against the stock forming the "cheekweld" in exactly the same place every time so that you are looking straight down the top-center of the barrel at your front bead. This is a lot like drawing a bowstring with an arrow knocked on it back to exactly the same point on your cheek every time in archery. However, fit is considered critical when selecting a shotgun and if you picked one perfect for you during conventional use, your head will be down tight on the stock and your eye will be too low to see into a red dot sight at a suitable angle. You've probably experienced this when trying to put a scope on a rifle that required a high ring height that forced you to raise your head up off the stock to look through the scope. To make the red dot work, you would have to add a rubber or leather cheek pad to the stock or shop for a new shotgun with a stock having a taller cheek piece.