School me on concentricity

[Tony Z]

Grant i saw the same eccentricity in the very first of the Lap 6.5/284 brown box. Blue box i have no idea about. After several firings the cases bent thick side short, thin side long. So badly that when chambered the bullet had scrape marks from entering the neck and throat cockeyed. No amount of sizing or general fucking around could get some cases to chamber to the point i was happy. Consequently it shot like crap. Really badly and was progressive. The older the cases, the more they were bent and the worse it shot.
Most people thought i was the crazy lunatic from NQ for condemning the brass to the trash. Five seconds on an internal concentricity gauge showed variations of 5 thou or more in wall thickness that was highlighted when skimming the necks with a turner. Conversely brown box 6BR and 308 brass showed virtually zero runout or eccentricity anywhere.

In my experience it takes a really gifted machinist to create an off center chamber or die. Misaligned presses are common. Handloaders with ten thumbs are even more common. Not saying the OP has ten thumbs
My point is that the seating stem like in Lee dies produces better alignment. Decapping rods with expanders are major culprits for bent necks. All needs checking. This is why BR shooters use inline dies with bushings. Otherwise invest in the Loc n Load concentricity gauge that can straighten loaded rounds. I have one and find it extremely good. I used the Forster prior to that.
There is a simple test here where you can take the most crooked cartridges and shoot them against the straightest. If there is an issue, it will show up. Unlike my old 284, really good leades and throats can correct some misalignment.