Make absolutely sure the beads line up correctly every time you monut the gun. To do this properly is close your eyes mount your gun as you would when shooting a target, now open your eyes and what is the sight picture? Now do this several times, close your eyes mount the gun as you normally would then open your eyes, is the sight picture correct. After 5 mounts this way you should know if your sight picture is correct for you.
If it is OK proceed to the 13 yard pattern sheet and draw a cross, now mount and shoot the gun it should print perfectly vertically, not to one side or the other, you should do this off a reast.
I agree with Pat Ireland and this will back up what he says, the above procedure should be your first step in setting up your gun.
If it shoots off to one side the Bbl needs to be corrected or if it is a choke tube model try na different tube and see if it corrects the POI.
I have done this procedure many times with many different guns to get optimum results.
Gary Bryant
Dr.longshot