I agree, it is interesting. And also find group location a bit of a surprise.

My take is the primers seem to have little effect on accuracy in this case. Throw out the first cold/clean barrel shot as being typically a little bit out of group and you find the first two groups are virtually equal, the the third group (on a hotter barrel) starting to open up a little.

If you do repeat the experiment, consider shooting one fouling shot then alternating shots with each primer. Shoot the first shot on target 1 with the STS, the second shot on target 2 with the 209A, and the third shot on target 3 with the W209. Then repeat the alternating sequence for shots 4, 5 and 6, then again for 7, 8 and 9. That should eliminate barrel heating as a factor.