I had a different experience, though it was clearly due to a specific set of materials. When I tried using 2520 in 308 in my M1A one year, the 100-yard bench groups grew from the 0.7 moa I had been getting to about 1.2 moa. I experimented around some and didn't have much luck until I deburred flash holes, and that brought the groups back to 0.7 moa. Some caveats: In that gun, deburring flash holes made no difference at all with stick powders, which all shot tightly without taking that step. The 2520 load did not fill the case well, which exacerbates ignition issues. At the time (early '90s) I didn't know using a magnum primer, both because of the extra space and because the older spherical powders are harder to ignite than stick powders, might solve the problem without flash hole deburring, so I didn't try it back then. So, bottom line, it can help with ignition consistency, but usually isn't necessary.
The flash hole size thing is interesting. A number of folks have tested and proven the use of small primers and small flash holes produce tighter groups. But there is also a study of flash hole size and centering you can find online that was done by a graduate student who got Fiocchi to give him unpunched cases to drill and the use of their universal receiver and test barrels. He conclude that of 1.5 mm, 2.0 mm, and 3.0 mm flash holes, the 3 mm (0.118") shot best. So, I don't really know. I'd have to do a lot of testing to separate the variables out.
Unclenick. I can relate to what you are saying. But may I offer another possibility. You know what I do and you know my site. All I do is test under as scientific ways as possible for consistency. On several tests, when changing the seating depth on an AR15 as much as 5k only (going from 2.255 to 2.250) the velocity changed, and the groups got literally 50% smaller. Repeatedly. So it was consistent. Such a small difference. Its because that powder is PICKY. It likes a specific area. AA2250 is very picky, and inconsistent, sort of like CFE223. ES is all over the place.
So what I am saying is, I cannot really conclude anything from testing with ball powders because they are picky and not easily repeatable due to their massive sensitively to the environment. Its sort of like testing out brass using 55 FMJ 10 cents a round bullets that are junk accuracy. You can't learn anything about brass when one of the components being used is wildly inconsistent. Or its like using a chrome lined barrel trying to best Berger match bullets and getting 1.5 MOA. You can't learn about match quality bullets and quality brass and quality anything, without using a quality barrel. When you put together quality "everything" and load to precision, I think some evidence can be presented that shows correlation to reach a reasonable conclusion.
This is why, when I conduct tests where I want to learn something, I only use very temp stable stick powders, such as AR Comp, or Benchmark, or Vihtavuori N133, or N135, etc, and neck turn the brass, full case prep, trying to get the lowest SD and ES possible so I can have sort of a control. Then when I get a stable control that operates consistently within a small range, I can change 1 element of it, and then tell you with correlation, the evidence of this. Here is an example of such a test.
http://natoreloading.com/brassprep/
Another one is this one, on neck turning..
http://natoreloading.com/Neck/
When I test across so many loads, and then look at overall, then run other tests like brass prep, it correlates with the neck turning test. It also is probably what you would consider common sense, as it just makes sense prepping brass producing more consistency.
I am not sure how to make the flash holes smaller, because I can only get brass with about a 80k hole in it. I don't have any access to brass that has no hole so I can try like 65k or something.
so back to the 308. I think by changing the dynamic of the flash hole, changed slightly, the pressure inside the case, and since its picky, I am guessing whatever it did, put it past the edge of the node. Its so hard to say, you know how complicated reloading is.
I look forward to sharing the results. I am really hoping there is zero difference, because that is ALL we all need right? 1 more thing to do with brass prep! it already takes long enough! haha