To better understand exactly how primer pocket uniforming helps keep your handloads consistent, it pays to understand primer anatomy. Primers consist of a small amount of explosive mixture
contained in a metal cup. Inside the cup, a small metal anvil projects into the explosive mixture, with its apex directly under the center of the primer cup, where the firing pin will strike. When
the firing pin strikes, it drives the outer skin of the primer down onto the anvil, causing the explosive to detonate, which in turns shoots a spark through the flash hole that ignites the powder
charge in the cartridge.
If there are differences in how high or how low primers sit in the cartridge cases, then there will be variations in how the firing pin strikes the primers. This will in turn cause slight differences in ignition of the powder, leading to variations in velocity that then cause shifts in point of impact on the target. Making all the primer pockets exactly the same depth and shape goes a long way toward eliminating any variations as early in the chain as possible - thus primer pocket uniforming is the reloading equivalent of nipping the problem in the proverbial bud.