Lucas, some comments on your remarks:
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You need to establish, for want of a better word, a datum line. I use a sinclair comparator tool. If you don't have one you stated that you have a lathe.
What you want is a bushing that goes over the neck and contacts the tapered part of the case halfway between the neck and the wall of the case. Using this bushing and a caliper you will be able to compare the datum line between a unfired case and a fired case. The difference in these 2 measurements will tell you if you have a head space problem occurring. they should be less than .005
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Good idea. I've seen folks make one that's exactly an inch long with a .400" hole in it for .308 Win. cases. That's the diameter of the datum point on a .308 Win. case and chamber. With a 4" caliper, it's easy to measure case headspace then subtract 1" from the reading. You can look up your cartridge shoulder datum diameter in SAAMI's web site.
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If you measure the full length sized case and compare it to the fired case those dimensions will show you that you bumping the shoulder back to much. I would think if this is your problem you wouldn't have that ring at the bottom of the neck. Your not sizing your neck for its complete length. Look at your die and see if there is a ridge or shoulder in the top of the neck sizing area. if so you may have to trim the brass.
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Also good info.
If one bumps fired bottleneck case necks back about .002", you'll get excellent accuracy and long case life. Sizing the neck all the way to the shoulder also makes it better centered on the case neck.
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Regarding that situation I don't see anything wrong with that. As a matter of fact its a positive because when you chamber the round upthe unsized area will help center the bullet in the barrel. I do a lot of neck sizing and only resize my necks 3/4 of the way. I am using the unsized area to center the round in the barrel to increase the accuracy.
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I disagree with this part.
All bottleneck cases headspacing on their shoulders center perfectly up front in the chamber. Their shoulder angle's the same as the chamber. When the firing pin smacks the primer, that drives the case hard into the chamber shoulder perfectly centering it. Even a .243 Win. cartridge will center perfectly up front in a .308 Win. chamber. It doesn't matter how much clearance there is between the case neck and chamber neck; rarely, if ever, does the case neck touch the chamber neck when it's fired. The firing round's case neck has quite uniform dimensional space around it when its centered on the chamber shoulder. Chamber necks are larger in diameter than loaded round case necks.
If the bolt's got an in-line ejector in its head, the spring in it pushes the ejector forward also pushing the chambered round forward until the round's shoulder stop against and well centered in the chamber shoulder.
All of this is why benchresters switched over to full length sizing their fired cases a few years ago. Other top accuracy folks have been full length sizing their fired cases since the 1950's.