Geezerhood,
Thinking about your problem, there is one thing that I found using group measurement tools.
The bullet size on the target often isn't the size of the caliber.
I find that for some lighter target papers, for non-wad cutter bullets, and for bullet speeds that are well below Pmax, the paper stretches around the bullet body and the resultant hole is smaller than the caliber diameter.
I print my own targets because I like to shoot at a white background with a small sighting point cross hair (keeps me from canting my scope) with a diamond on top of it where the rounds will hit so my aim point will not be destroyed. I use a high powered scope (30X t0 36X) when shooting targets to test loads and measure groups. The weight of paper makes a big difference in the final hole size. 20 lb. paper stretches a lot, but I have found that 80 lb. paper is much better.
For example, my .308 tipped or HPBT bullet holes measure closer to 0.277 than to 0.308, the 6.5mm bullet holes are closer to .243 than .264 and the .223 bullet holes are closer to .204 than .224.
Interestingly enough, the .22-250 bullet holes are just about at .224, probably because they are moving faster than the .223s.
My .22LRs are closer to .177 than .224, but they are moving half the speed of the .223s.
Center to center measurements shouldn't be impacted by caliber but you might try understating the caliber to see if that helps.
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