In order for the gun to hit point of aim, the front sight and the rear sight must be the same height above the bore. In the case of a bead sighted shotgun, the rear sight is the top of the receiver, and you can see the receiver is higher than the barrel. Since the barrel tapers towards the muzzle, if the bead is mounted directly on the barrel, it effectively makes the bead even lower, relative to the height of the receiver. If the bead is not raised to the height of the receiver, the gun will shoot high, when the shooter's cheek is in contact with the comb of the stock. If you don't want to modify your barrel, you can adopt a shooting technique, where you shoot just as you let the bead drop below your view, but it takes practice to make it intuitive. Another technique, which I expect many shooters use anyway, is to use jaw bone contact with the comb, instead of a cheek weld, effectivley removing the receiver from the sighting formula.