There is not much I can add that has not been said. My favorite theory was mirage until that was shot down. It is still hard to believe that mirage doesn't really move the bull after watching it bounce around in the scope like a ping pong ball dropped in a sink.
I did run the numbers for the load listed above and all else remaining constant, raising the temperature from 70 to 100 will raise the point of impact 1.1" at 550 yards. On average, the humidity does not increase with increasing temperature, so that normally would not be a big factor.
A crosswind will also cause vertical deflection. I do not have numbers at hand for the 68 bullet, but just as an example here is the 168 MatchKing;
The 168MK, fired at 3050fps, from a 10" right hand twist barrel, will deflect 30.3" at 600 yards in a 10mph wind. It also will deflect -2.6" if the wind is from left to right and +2.6 in a right to left wind.
So, rather than one explanation, it is looking more and more like a combination of several smaller things. To insure that first shot in the 10 ring, there are a lot of things to consider.
Jack
