I just got the Nikon 2x7 Prostaff shotgun scope and mounted it on a combination shotgun (12 ga/.222) but I can't seem to understand something. When I sight the rifle in at 50 yds dead-on it is off high right at 100yds, when I then sight it in at 100yds the 50 yd group is off to the left usually 2" or so. Is this a problem with a shotgun scope that I should expect? Help needed!

Jim