While we are on the subject, I will relate a story about one of the tricky ones.
I had a Savage .220 Swift, and that caliber has a rather small shoulder angle. Cases of that design can be more easily driven forward by the combination of the striker fall and the primer detonation. I had gotten my Stoney Point Headspace gauge by then. Luckily, I was working on a small set of cases, that I FL sized with an RCBS FL die. I if I remember correctly, I only bumped the cases a thousandth or two, no more. I am sure of that. Anyway, after firing the FL sized cases, I noticed a bright line, and upon inspection with straightened paper clip, with a short bend at one end, felt a slight thinning on the inside of the case, where the bright line showed on the outside. Thinking about this a little, I decided that with the shallow shoulder angle, by bumping the shoulder, I had given the case just enough room to "take a run at" the chamber shoulder, so that it could be driven farther forward than it otherwise might have been, sort of like having some room to swing a hammer. In any case, for the next batch, I set the die so that the shoulder to head measurement was the same as they came out of the chamber, and the problem was solved...no more bright line. Thinking about it some more, I decided that cases with these sorts of shoulder angles are unlikely to be able to come up with much resistance to bolt closure at their shoulders, and that if there is any, a slight reduction of the body diameter, combined with keeping the shoulder as it came from the chamber will make the cases chamber easily.
Another thing that I learned, that relates to this came from fire forming and reloading 6PPC cases at the range, with a small set of cases. It takes several firings for a case to reach its maximum head to shoulder dimension, so I usually fire one case several times, in a particular barrel, to have an example of the maximum dimension, and it is in relation to that, that I bump. If I don't have such an example case, and need to FL size once fired cases, that are out of the rifle that I am loading for, I simply set the die so that the shoulder is the same as the fired case, and test it in the rifle, working with once fired, I have never had this result in a tight bolt.
Getting back to the Swift, back then, if you sent them the die and a fired case or two, RCBS would lap out the body of the die for minimum sizing. As the die came, it reduced the body diameters more than I liked, and because of that, I had to trim quite often, taking quite a bit off of the length of the case each time. After having the die lapped out, the amount that cases grew from FL sizing was cut in half. Taking this to an extreme, my custom 6PPC die does not change the shoulder diameter by a measurable amount, and it sizes the back of the case by half a thousandth. I usually set shoulder bump for a thousandth or less, and as a result my cases hardly grow at all, and I do not use light loads.
This has rambled a bit, but for the newer reloaders, I hope that there has been something useful.
