FortHunter - be aware that simply chambering a rimless cartridge into a chamber and firing it, will likely not "fire-form" it properly - there is nothing to hold the case firmly back to the bolt face - the firing pin will push the case as far forward as it can, before setting off the primer - meaning, your case shoulders will be slightly shorter than the chamber shoulders - if the case head did not separate from the case. You can read of guys seating bullets very long - to be hard into the rifling, as a way to hold that case back to the bolt face when firing (you probably want to reduce your load to do that!) and others will mention to oversize the neck and then bring it back down to fit your chamber - in effect creating a "false shoulder" that will prevent the case from moving forward during the firing pin strike - I think that case neck is typically thinner material than the case walls near the case head - so during the firing sequence, it is probable that the case neck and front part of the case will grab tightly to the chamber wall and hold it (due to pressure rise) and resulting in the case head getting pushed back solidly to the bolt face - either the brass case walls will have stretched, or the case head will have seperated, or nearly so. Then, subsequent Full Length resizing that pushes the case shoulder too far back will simply repeat that process, until one DOES get case head separation.
It is a similar issue with belted or rimmed cases - it is sometimes desirable to transfer the headspace to the case shoulder - not to rely on the case rim or case belt for headspace control - to resist the forward movement of the case due to the firing pin strike. Make your cases fit your chamber, and the whole issue of incipient case head separation goes away (mostly).
As a "home done" check - so normal computer printer paper is circa 0.004" thick - you actually want about half of that clearance between case shoulder and chamber shoulder - 0.002"- MAXIMUM. So, set a case on a piece of paper and draw a circle - cut that out or be slightly smaller than the case head. Insert that between the bolt face and the brass case - if it chambers (bolt locks shut) you have circa 0.004" excess clearance - about double what you want to have, for maximum accuracy. When barrel chambers are cut, they are typically using a GO gauge (minimum length) and a NOGO gauge (often 0.004" longer than minimum). for best "accuracy", you want to be closer to GO than to NOGO. There is usually a third gauge - called FIELD - that is MAXIMUM length - so a chamber between GO and FIELD is "safe" to fire, but not necessarily the "best" fit for "most accurate".
