strange nobody mentioned checking the vertical alignment and tracking . ie shots high and right above the aim point and low and left below the aim point through the trajectory curve.
Also shooting the box , how many miss those out just because you just shot a little biddy group centre v bull, does not mean the gun is correctly set up !
This matters if you hold off or dial! A simplified way to do a quick check is use a plumb bob to scribe a line vertical with a sprit level and shoot one shot as high as you can pointing at the v bull and one low shooting on the v Bull, Are those two splitting the line or do they cross over ?
One minute of angle is approx. one inch near enough at just 100 yards and that matters
setting a zero in theory is one shot, checking it takes a good few more