I did something similar with Redding to what Bart did with RCBS, though not as soon as Bart did it. I suggested they switch their steel lock rings from a diamond knurl pattern to a straight knurl, and even gave them the right knurl size (turned out a metric pitch worked best at their lock ring diameter) to give them either 71 or 72 "teeth" (what the US coin mints refer to as "reeds" around the edge of a coin). Since about 71.4 teeth would correspond to exactly 0.001" die adjustment per tooth of rotation, either count is close enough to make adjustments on a loading die.
I still think either that or Bart's imprint are good ideas. I just figured that since they were knurling anyway, that wouldn't add an operation. They just had to wait until their old knurls were in need of replacement, then replace them with the metric pitch knurl I identified. Redding thanked me for the suggestion, and that was the last I heard of it.
I think James probably mistyped and meant a 0.05 mm feeler gauge (about two thousandths). If you cut your own from shim stock you can make a good fit, and even steel shim stock is soft enough that you can clamp it between two thin steel plates to hold it for drilling either a ¼" or a 6.5 mm clearance hole for ejecting primers through, should you need such a tool.
The confusion for me centers on the question, why do the cases need their shoulders set back more than the FL die normally does it? James said he normally resizes in a full-length die and trims and shoots. So it sounded to me like the FL die was already setting the shoulder back more than enough to fit his chamber, and that James had maybe misunderstood the posts on setting shoulders back to be referring to setback that was in addition to what the FL die already produces. That's backwards, in a sense, as the accuracy idea is to reduce the amount of setback normally produced by an FL die to just that which is required and no more. That's what extends case life.
If you have a short chamber or if you pick up brass that was fired in a chamber that is longer than yours, then the FL sizing die may fail to set the shoulder back enough for your chamber in one pass, and then you have the need to use a feeler gauge or a short deck shell holder or a die with its mouth ground back. In the case of long brass from another's chamber, you usually only have to do that extra sizing and setback effort one time in the life of the case. Once the brass has been fireformed to your chamber, you normally no longer need to use the special tools.
The only potential exception that comes to mind is one mentioned earlier, and that is if the brass has work hardened enough to become very springy at the shoulder. This is unusual as too springy to change size in one direction is also too springy to change in the other direction, but with a large enough chamber it can happen, and I've had cases the refused to hold onto a bullet because of it. If you have this issue, it's time to anneal the necks and shoulders to stress relieve them so the necks don't split and the shoulders set back more easily.
If you've never annealed before and you don't intend to do it more often than necessary, I recommend you start with the candle flame method because, though slow, it is almost impossible to overheat the brass with it. You just hold the case head between your thumb and index finger and roll it back and forth in a candle flame playing on the neck and shoulder. The rolling motion is to spread the heat evenly and the soot tells you if you are missing anywhere. Once the temperature gets too high to keep hanging onto the case any longer, you slap the case down on a damp towel while drawing and turning it over the towel to wipe the soot off, then leave it there to finish cooling. It's a slow process, but it works. When you get used to it, you can learn to use a torch flame, which is faster. But your timing is more critical with a torch because it can overheat and over soften the brass. It's not uncommon for overheated annealed brass to require re-annealing every three to five firings to avoid neck splits. Proper annealing can get you a dozen or more reloads without neck splits, though this does depend on your exact chamber and die dimensions and how close they are.
With his home made gauge, assuming it is similar to the improvised spacer types that Bart and I have posted our photos of before, James should be able to make the comparative before and after shoulder position measurements to determine shoulder setback in his die. Buying the Hornady comparator adapter kit is a convenience, as it covers a wide range of chamberings, but if you are shooting just one chambering, or if you are living where reloading tools are extra expensive, as James is, it makes sense to make or improvise your own. The idea is to find what size the cases come out of the chamber with, and then size the case only enough to set the shoulder back another thousandth. Two thousandths is usually used for a self-loader, but unless you are shooting a rapid fire pace from the bolt gun for matches or special training purposes, you don't really need that extra setback. It is just used for extra rapid feed reliability insurance. The need for it is probably an overabundance of caution in most instances. If take a case that has sprung back 0.001" and take it back another 0.001", you already have a 0.002" difference, and that is all the difference there is between most new .308 Win ammo and a minimum chamber.
I would not recommend the Forster Bump Die for what James is doing as it only moves the shoulder back and does not narrow the sides of the case. You actually have to do both if you want the case free to self-center in the chamber, which happens when the case shoulder is driven into the chamber shoulder by the firing pin. There has to be some sideways clearance for that to work. You typically don't get accuracy that's quite as good, otherwise.
There are exceptions. If you can select cases that are absolutely perfectly symmetrical inside and out, so they expand symmetrically under pressure, and you have a chamber that is perfectly symmetrical about the bore axis so heads don't fireform out of square, and you have a bolt face that is perfectly square to the bore axis for the same reason, and that enjoys full and even bolt lug contact when closed, then neck sizing only will give you accuracy that is theoretically going to be as accurate and it will give you the longest case life. The problem is that first step. Finding perfect cases that won't expand more on one side than the other and move the neck off-center for the next loaded bullet is the main problem. If I find more than a dozen out of 1000 measured cases, I feel lucky.
