OK--THAT was a fascinating read. Many thanks for posting this info!
My own far-less-sophisticated observations over the years have been that primer brisance/strength/heat propagation is a lot more critical element than we give it credit for being. I didn't give it much thought until about 10-12 years ago when rassling with flyers in my 25/20 WCF and 32/20 WCF rifles. My Nevada Cast Of Cast Bullet Characters were having a not-too-deeply-irrigated discussion about the Slings And Arrows Of Outrageous Fortune regarding flyers that spoiled groups.....those not related to the Nut Supporting The Stock, more specifically. Urny stated that he had seen immediate group shrinkage with his 22 Hornet/Ruger #3 through the use of Remington #6-1/2 primers. He explained that Remington makes two small rifle primers--the #6-1/2 for cases sizes like 22 Hornet, 218 Bee, 25/20 WCF, 30 Carbine, and 32/20 WCF--and the #7-1/2, for the 222/223-sized cases. The #6-1/2 uses a rifle-pressure cup but a smaller primer fuel element tailored to smaller case capacities and powder weights.
Such things are called CLUES where I once worked. I suspect (but am not certain) that this meet-up was during one of the Big Reno Gun Shows I showed up for. Folks, I'll visit the United States any time I can get away to do so. I do recall picking off a sleeve of Rem #6-1/2 primers right after this discussion, and I still have about 1500 of that 5K acquisition on hand. Remington primers are largely unobtainium hereabouts, so another visit to the Miwall table at the reconstituted BRGS is called for. Aw, shucks.
The results were immediate and conclusive. Jacketed or cast, flyers were largely eliminated and group sized shrank SIGNIFICANTLY. Velocity ES and sd were significantly reduced--across the board--22 Hornet, 25/20, 30 Carbine, 32/20--rifles and handguns. This correction was one of the most significant improvements I have seen in my reloading outcomes in all the time I have been involved in this madness of ours. Primer selection definitely matters--A LOT.