OT is correct, I just wanted to add a couple of comments.
Originally Posted by Otintx
Case length in a straight-walled rimless pistol cartridge (.45 ACP, 9mm, .40 S&W, etc.) is, or rather should be, a critical dimension because it determines headspace, which in this case is the distance between the rim of the case and the breech face. Since the case is pretty much captured between the breech face and the square shoulder at the front of the chamber when the pistol is in battery, I don't see how it could stretch in length, but I'm sure they do. If the case stretches at all, I think it must happen after unlocking, and not while the pistol is in battery.
(Illustration courtesy of Wikipedia)
My experience with pistol reloading is mostly .45 ACP, which is a low-pressure round, so I'm still learning. But having loaded about 3,000 rounds of this ammo, I think I have seen perhaps twenty or thirty that failed the Chamber Checker test, where the cases apparently had grown. I put those aside for practice, almost hoping that they would cause a feeding malfunction to give me some exercise, but that hasn't happened yet. As long as you check ALL of your ammo with a Chamber Checker, you will catch anything that has stretched.
The occurrence of case stretching in .45 ACP, in my experience, is so rare that I have stopped measuring case length, which is something I did when I was just starting to load pistol. I just catch them with the Chamber Checker after reloading and set them aside.
As OT inferred, a bottle-necked cartridge, like .357 Sig or most modern rifle cartridges (.223, .308) takes headspace from a datum line on the shoulder of the case. There is room in the chamber forward of the case mouth to allow the case to stretch, and they do. I personally check the length of every single .223 or .308 round that I load and trim them carefully, because uniform case length has to be maintained to achieve peak consistency and accuracy from ammunition. (information source: NRA Firearms Fact Book, Third Edition)
Until I read OT's post I didn't realize that .357 Sig was bottle-necked (because I have never fired a .357 Sig), but I believe that everything I wrote about rifle cartridges here probably applies to the .357 Sig, and that if you reload it you should probably check length at least every third loading. The ones that are too long will need to be trimmed. In fact, you probably should trim the whole batch if any of them measure beyond the spec. I may be way off about the frequency of checking that is needed here, let's see if somebody reads this and corrects me. It's clear that .357 Sig is a high-pressure round, since it typically runs at over twice the pressure of the .45 ACP. (information source: Hodgdon loading data website)
Purkeypilot, I expect we are throwing around some terms that are going to be new to you, don't be surprised. Allow me to recommend a couple of sources for you:
Sierra Bullets Reloading Manual
This is a little spendy at about $65, but it is really complete. Although the ballistics are all based on Sierra bullets, there is a lot more to this book than loading data.NRA Firearms Fact Book
, Third Edition: this appears to be out of print but can still be bought new or used at Amazon.com. A good replacement would be the newer NRA Firearms Source Book
Again, this is a bit spendy, but I would not be without a copy of this book.