Opaww is correct the small aperture is for long distance and the large aperture is for close quarters or close combat. As far as the dot. I think you will vastly improve your group if your rifle is zeroed at 50 or 100 yards. Then draw a Black + (Cross) on the target paper with the lines being about 6 inches long (each). Then hold the top of the front sight post on the bottom of the Horizontal Line and centered on the vertical line. The width of the Cross will depend upon the distance. For example at 100 yards make the lines on the Cross about 1 inch wide using iron sights, or what ever is the thinnest you can see well? obviously the closer the target the thinner you need to make the line! I am betting this will help you to shoot much better groups.
When using a scope or optic you can make the lines much thinner due to the fact you can see them better.