I have observed the same. If I take three firearms minimum to shoot, and rotate the every three shots(with my first in every three being at a site-in target then two in the group), my groups are better. It's a real pain when your trying to develope loads, cause I don't like to be loading a bunch of different calibers and charge weights, bullet weights etc.... to much for my brain to keep in order. Plus to do a good job on testing, it takes more time than I have.
How many do you load to test?
RV