Yesterday I zeroed my rifle (Tikka M595 .243 w/wildcat mod, S&B 6x42) for a stalk I have booked next week. fired 3 shots at 100 meters, formed a neat triangular 1" group in center of target. I thought, seeing as I had plenty I've shoot a few more groups for fun. Each group I shot after that got increasingly sporadic. The next one were all on the same horizontal axis but where 2" and 3" respectively left of center, the next group where even further from each other and included divergence on the horizontal as well as vertical axis.
I am thinking something bad is happening when the barrel has heater up a little - and it's time for a trip to the gunsmith, or is there an easy diagnosis and fix a novice can do from home? a tenner fits around the barrel when cold - forgot to try when warm.