The first time I read "The cartridge should fit the chamber like a rat turd in a violin case" was at the old German Salazar's The Rifleman's Journal web site. And it means the case is full length resized and the body of the case does not touch the chamber walls. Meaning the case give the bullet "wiggle room" to be self aligning with the bore.

Below are Kevin Thomas and Erik Cortina of Team Lapua USA






Below Mr. Salazar is responding to a question about "partial full length resizing" vs complete full length resizing. And the benefits of how full length resizing helps accuracy. Keeping in mind that a case with unequal case wall thickness will expand more on the thin side of the case and warp and become banana shaped. And a warped neck sized case can push the bullet out of alignment with the axis of the bore.


Reloading: Partial Neck Sizing
by German A. Salazar
http://riflemansjournal.blogspot.com...ck-sizing.html

"Now the last scenario, a full-length sized case in which the neck is also fully sized. There is clearance at the neck and in the body of the case, the closest fit anywhere is the bullet in the throat. If the neck to bullet concentricity is good (although it needn't be perfect), then the bullet will find good alignment in the throat and the case body and neck will have minimal influence. Let's not forget that the base of the case is supported by the bolt face or the extractor to a certain degree as well; this is yet another influence on alignment. As you can see, there are several points from base to bullet that can have an effect. My procedure is to minimize the influence of those that I can control, namely the case body and neck, and let the alignment be dictated by the fit of the bullet in the throat and to some extent by the bolt's support of the base. Barring a seriously out of square case head, I don't think the bolt can have a negative effect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber. Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway."

"AND" at the Whidden custom die website they tell you they get the most concentric cases with non-bushing full length dies. This is because any time you reduce the neck diameter .004 or more with a bushing die it can induce neck runout. Also a neck sizing die does not fully support the case body, meaning the case body and case neck are not held in perfect alignment within the die. Bushing dies work best in custom tight neck chambers with neck turned brass where the case neck does not expand over .003 when fired.

Ultimate 6.5 creedmoor sizing die comparison