ranger, that's an interesting coincidence. We started reloading the same year, 1965.
I used conventional sizing dies for most of those years. I also use neck size dies for a couple of calibers, and for others, I simply back the sizing die out one turn and use that to neck size. Neck sizing only works if your gun has a tight lock-up and the loads are fairly mild. That's no criticism of neck sizing, only an observation that it doesn't work all of the time. I realize that you know that, but I am only mentioning it for the newer reloaders.
As for the X Dies:
There is no physical way that pushing back on the mouth of a rifle case can restore or re-thicken the stretch-thinned brass above the web. The pushing back effect will simply stop at the neck to shoulder juncton.
The neck is not pushed back into the shoulder junction. The top part of the case looks (and measures) the same after sizing, as far as length goes. The diameter is reduced. The "pushing back effect" DOES NOT stop at the neck to shoulder junction. The case length difference is at the web.
With full power loads, particularly those fired in semi-automatic rifles, my brass usually fails first at the web. I get the characteristic bright ring around the case at the web, and with a bent wire probe (a reformed spring hook) slid along the inside of the case at the web, I can feel the thin area of the web at the same point on the case as the bright ring appears on the outside of the case. That's time to discard the case, because I know I will get case separations if I continue to reload those cases, even after conventional resizing and trimming to length.
Using the X Die, I can generally reload the case about twice as many times as with the conventional sizing die. They still thin at the web, but just not as fast. In fact, case mouth splits become a factor when the brass has been fired enough. With conventional sizing dies, I toss the cases because of thin webs and incipient case separations (and actual case separations if I load them one or two more times) long before the case neck splits show up.
One of my M1As seems to have a "springy" action, although the headspace measures within specs. Cases fired in that rifle will develop the bright ring at the web after only two or three reloads with conventional dies. With the X Die, I get about twice as many reloads before I toss the brass. I have had enough complete case separations with that rifle using conventional sizing dies (also RCBS) to know not to continue to use stretched brass. But again, with the X Die, I get double the case life.
I can see the difference in the bright ring at the web using the X Die. I can also feel the difference on the inside with the spring hook. I have cut stretched cases lengthwise and noted the differences on the inside of the cases that have the bright ring.
My personal experience has been that the X Die extends case life. The difference is at the web. There is no difference at the neck to shoulder junction, and the neck is not pushed back into the shoulder.
I will continue to use the X Dies for those calibers for which I have the dies. Maybe others will have different experiences, but I recommend the dies.
As I said before, I don't work for RCBS, and I have no financial interest in the company (other than that I have bought a hell of a lot of their dies over the years). I wouldn't try to talk anyone else into using the X Dies.
I do recommend them. After that, each reloader has to make his own decisions.
Thanks for your courteous replies.
Jim M