Re: Advantage of case concetricity? NONE
Concentricity will always be important to some shooters, and for some shooters it will help help their performance to a useful degree.
It will help shooters with SAAMI chambers less than those with tight necks. Where group size meaurements down to the second and third decimal count, it will be important.
I resize differently from most, using a standard F/L resizing die and neck sizer ball. I back off the die several turns, leaving a significant portion of the neck (and a similar length of the lower case wall) untouched.
This expanded portion of the case wall and neck helps align the case more concentric to the chamber's central axis when the case is inserted completely into the chamber. To some degree, this addresses the OP's original concept.
Another technique is to do my final die lockdown with a case inserted and the press at full stroke. This will help get the die as close as I can get it to ideally aligned with the case.
I figure that going beyond these basic steps just undermines my confidence in my ammo and my handloading effectiveness, and introduces issues that I neither want to deal with nor honestly believe are going to have a significant benefit, given my own marksmanship frailties.
In the end, the problems we have are quite often of our own making. I see it as a question of priorities. I go after the big issues and settle for whatever smaller ones remain.
As for the question of why handloaded ammo is more accurate then factory, I am willing to make the charge weights more consistent. The rest is a matter of case prep, which I would do in any case, and some attention to neck tension (which is at the foundation of how and why I do partial length neck resizing).
Greg
