In the latest edition of the Black Powder Cartridge News, Fall 2021, Rick Moritz authors one of the clearest descriptions of how the rifle barrel vibrates while the bullet is traveling to the end of the muzzle and the effect it has on the bullet impact down range producing the least vertical spread.
I use a close variation of the load development method Rick describes and it is good to see it laid out and written in a very easy to read and understand essay.
Check it out.
Michael Rix