Boresighter

You get a new rifle and optics and head to the range. You aimed the target at 100 yard and pull the trigger, so exciting! But wait, where is the bullet hole? You set the target back to 50 yard, shot again. Still, there is no bullet hole. You move the target back to 25 yard and finally on the paper. It's 5 inch high and 6 inch to the left, no wonder.