Multi part question here, so let me try and take these one by one. In re the S/B dies, amny shooters assume that the standard F/L dies resize a fired case back to its original, unfired dimensions; this isn't the case. Most makes leave them a bit larger than new, unfired brass, though it's pretty close. Different die makers also vary a bit in where they choose to run their tolerances. Redding only began offering S/B dies a few years back, as they were already cutting their F/L dies down to very tight specs. On the flip side, many years back when I was still on the Sierra tech line, when a caller called in and reported problems that dealt with failure to chamber issues, we'd walk them back step by step in trying to eliminate the problems one by one. In those cases where it came down to a bad die that was simply failing to resize the cases enough, the dies were almost invariably Lee dies. Without fail. Obviously, Lee was willing to run their production on the upper size limits, and it occasionally caused problems with some chambers on the market. The S/B dies just manage to sidestep a lot of this nonsense, and get straight to the heart of the matter. Some of the others were probably in between these extremes, but since the S/B dies are more commonly available now from everyone, I just think it's good practice to avoid the problems in the first place.
As far as the brass itself, while many will claim that the S/B dies are "harder" on brass than standard F/L dies, they do work it a bit more, but we're talking about a very minor degree here. Besides, the issues that usually cost shooters their brass is case head separation, which is in turn caused by bumping the shoulders back to far. Many shooters seem to assume that the S/B dies also bump the shoulders back further, but this is the responsibility of the shooter. Use a gage to set the dies up, and for the ARs you'll want to bump the shoulders back by .003" or so, a thou or two more than you would for a bolt gun. If the S/B dies were truly causeing shorter brass life, we'd see lengthwise cracks running up and down the body of the case, as opposed to the circumferntial cracking we see being caused by headspace issues. Two different things entirely.
In terms of parts wear, I have yet to see any studies done to failure to demonstrate this one way or the other. Such a study would be well beyond the time and expense of the average handloader, and the military certainly has no concern whatsoever about reloadability issues of brass, nor lifespan of parts with various types of reloaded ammo. Basically we're on our own here. But the advice concerning parts wear and easier extraction came directly from William C. Davis Jr., who I'll admit to shamelessly parroting here. If you're going to pass on such commentary, he's the guy to get it from, and I'm comfortable in doing so. He describes this is some writing he had done for the old NRA book simply titled, "Handloading." Long since out of print, but worth its weight in gold. A real treasure trove of some obscure knowledge in there.
If I missed anything, just let me know and I'll try to refine it a bit.
P.S. the S/B die refers to a "Small Base" die, not a "Short Base" die. As I mentioned, headspace is an indepenant variable, which should be the same between S/B and F/L dies.
