POI

hi help i seem to be having what seems to be an age old problem with there airforce/gunpower gun range i fitted a newly machined bsa 18″ 177 barrel in my rifle chronoed it til i was happy with t numbers shot to shot was about 3 to 6ftps differences was more than happy using Kodiak’s the rifle was roughly zeroed to 40yards then wanted to see if the groups had tightened from the standed 12″ barrel low and behold they were unreal at last i was getting a single ragged hole from a 5 shot group until i noticed that if my head wasn’t in the exactly the same place the flyer’s started which you would expect as poor repeat mounting will give u poor results i have never had this problem on any of my other wood stocked rifles due to better cheek, scope alignment so has any1 any ideas????????????????????????? 🙁