I have a Colt AR-15 target rifle with a Colt 3x20 scope. I don't have any instructions for the scope and would like to know what one of the adjustment screws, in fact, adjusts. It is on the right side of the scope and is the only adjustment with a screw-on cover. The adjustment itself uses a slot screwdriver to change it. Could someone tell me what this adjusts? Thanks in advance.