Perfectly fair question, no foul there at all.
In cases that actually needed them, the belts served their intended purpose. The belt stopped the case from moving forward in the chambers, when the shoulders were too shallow to do this reliably with the machinery then in use. Also have to remember that most of these were originally loaded with cordite, which is why the cases couldn't be formed any differently. Those sloping shoulder were the result of straight-walled cases that were loaded with cordite before being necked down to their final form. In otherwords, they couldn't make the sharper shoulders like we have on later cartridges at that time. So, the belt acts in exactly the same manner as a rim on a 38 Special or a 45-70.
With the newer magnums (and the use of granulated powders), the more pronounced and sharper shoulders became viable. Better machinery, which could control dimensions better, along with better metallurgy in the material itself allowed the use of the more modern case design. With these shoulders, the cases CAN be headspaced on the shoulder, making the belt completely pointless. Worse than that, the belts are actually a weak point, and can cause several problems. Case separations are one of these, because of how we've been told to set up or dies. Don't get me wrong, dies have to work for all chambers, and the manufacturers do a pretty good job of ensuring that they do. In the case of the belted magnums, however, they generally leave more than enough room to push the shoulder back far more than it should be. Since the case will still headspace on the belt, the rounds will still fire reliably. The problem is, that gap that now sits between the case shoulder and the shoulder area of the chamber. Everytime that case is fired, it's going to expand to fill that chamber by pusing it forward. After just a few firings, you're right back to where you'd be with a conventional rimless case, if you were to set the shoulders back too far (excessive headspace), and you wind up with head separations. This is what you get when you follow the (some or most) die makers instructions to run the die down to where it contacts the shellholder and then load it to where it cams over when bottomed out. The better way to do this is to (as I said) treat these belted cases as if the belt wasn't there at all, and simply headspace it off the shoulder. No case stretching issues, as the gap that would otherwise be between the case shoulder and chamber shoulder, is now a gap between the forward edge of the belt and the belt ledge on the inside of the chamber. Firing won't cause the case to try to fill that, since the forward movement is stopped by the shoulder, so the brass lasts longer. You've got the right idea here, but you're mistaken about where the brass does its stretching; it's still back by the head, just forward of the belt. Case forming is a bit of a specialized situation, and it's one that we're not repeating each and every firing. One shot and formed, and from that point on it's no different than a case that was originally drawn this way.
Belted cases are a holdover, and are still found on more modern case designs like the 300 WM and 7mm RM because shooters of that period simply wouldn't buy a "Magnum" case that didn't have a belt. It was pure marketing BS, plain and simple. The cartridges didn't need them for functioning, and they don't make the case any stronger. Actually, they weaken it from a mechanical standpoint. I'm pretty pleased to see that most of our newer magnums have finally dispensed with the belt, since this is a huge step in the right direction.
