I've read all of the posts and unless I missed it I don't think anyone mentioned the possibility of copper fouling in the barrel. Have you considered it? If you've shot 1 1/2 boxes 3 times that equals 4 1/2 boxes. Or was the 1 1/2 boxes spread out over the three trips to the range? I was having the same problem with my .280 last year. Boyd stock, Mauser action (copy), 26" Douglas barrel. At 100 yards it was shooting softball sized groups which is good enough for a deer rifle (for some) but I could never get it to hit dead center of the target. It was also vertical stringing to the left and right. Everything you checked I checked and was about to replace the barrel before a long time shooter suggested checking for copper fouling. Bingo! Ran a patch with Barnes CR 10 (I think) through the barrel and it came out blue green. After a good cleaning I took it to the range and shot a ten shot group, dead center, at 100 yards that measured 1.342". Problem solved
Beautiful rifle by the way!