Hello, Frank and all,
Disclaimer: Pelletgage is my product, and it is designed to be an accurate and repeatable tool for determining pellet head diameter. I don't intend this as a sales pitch, though.
Since you have successful experience with a particular pellet, accurately determining the head diameter of those particular lots of "good" pellets is a starting point. My first thought is that there is excessive head diameter variation in some of your tins.
In my target shooting experience, and based on customer feedback, JSB is a good choice. They form cut lead wire chunks into spherical shape, weight sort, and use lot controls and managed die sets for swaging, and they have both manual inspection and test firing from bench rifles to produce a quality pellet. They make all the AA pellets, and it is likely that other controls are imposed on the production of these premium pellets.
While I've seen tins of my favorite JSB Exact 4.52 mm 10.3 grain pellets where almost every pellet is the same diameter (as determined by Pelletgage), I have also seen tins where some pellets vary 0.03 mm from mean, and tins where the mean diameter shifts by 0.02 mm.
So, a starting point is to get some of those "known good" tins (or lots) and determine the head size your gun prefers precisely.
While micrometers (and in some cases calipers) may give you a diameter, consider that typical accuracy of good calipers is +/- 0.001 inch, which is 0.025 mm. Micrometers are up to 10X more accurate. If you use these, skill and great care will be needed for valid results, and only one chord across the sphere is being measured. I believe some efforts to correlate head diameter with group size have invalid findings if the measurement method is not well controlled.
My own thoughts are that a "good" tin of pellets should:
1) not shift mean head diameter more than 0.01 mm from the desired nominal size
2) have 95% (or more) of the pellets within +/- 0.01 mm of mean size
While the idea of sorting all the pellets in a tin may seem onerous, sampling 50 pcs will statistically verify this, and is not so time consuming. To confirm the above, not more than one pellet should exceed the limits, and the mean size should be within 0.01 mm of the desired size.
Your gun may produce nice groups with say, 4.51 mm pellets, and again, with 4.53 pellets. However, testing might show that there is a POI shift in the grouping (a pretty common claim). In such cases, groups resulting from shooting a mixed batch of pellets is larger, and really a combination of two better groups.
Cliff Tharp published a (now retired) BLOG about his experiences as an airgun varmint hunter. He had issues with pellets, and went to some lengths to find the causes. In one instance, tins of pellets with a lot of skirt damage were tested, and the findings were a surprise.
I believe there are other possible factors in play.
During swaging, a punch shaft presses the lead sphere (or wire chunk) into the die. The result is a cylindrical cavity in the back of the pellet. There are instances where this cavity changes in position, depth, and even diameter. For "bolt action" loading, the probe of the loading mechanism may push the pellet into the leade at varying depths.
Any swarf or imprecise parting leaving residual material on the skirt margin can affect rotational inertia, and affect ballistics. I've seen a few tins where this is an issue.
An inexpensive headband magnifier with 2-3X power is very useful to look for physical irregularity.
Now, all the above may not have much validity if you are not shooting an accurate rifle in a very consistent manner - a challenge for all of us. I have had perhaps ten customers reply to me that they were gratified to find that pellets were the cause that had them adjusting/re-mounting scopes, cleaning barrels, questioning their own vision and hold, and then discarding some tin(s) that exhibit the issue.
There are some other notes you may find useful at the Pelletgage.com Facebook page. Good luck!