I had something similar happen with my 270. I distinctly remembered my cousin shooting a 3-shot group with all the bullets nearly on top of each other at 50 yards to the extent we had trouble determining where all three impacts were. Trouble was, I couldn't remember which bullet I was using - 130 grain Interlock, or 130 grain SST - nor could I remember the seating depth - to the lands, or the loadbook recommended depth - nor did I write the details down anywhere.
Later, when I tried to test some loads with the SST, none of them seemed to shoot better than 2.5", but I knew from past experience it was capable of MOA with factory Remington Core-Lokts and Winchester Power-Points. Tried the Interlock, same dismal results. I finally decided to test both bullets with both seating depths with a charge of 51.5 grains of IMR 4350, which I remembered was the charge I had used with my cousin and was safely below maximum. The results went something like this:
SST, seated long: ~2.5" at 100
SST, seated standard: ~2.75" at 100
Interlock, seated long: ~2.75" at 100
Interlock, seated standard: ~0.9" at 100
I learned that day that seating close to the lands does not always improve accuracy, and I also learned I need to WRITE $#!T DOWN!