Joe,
Did you read this part of my post? "... .30-30 (rimmed) lever actions have extractors. .30-'06 (rimless) bolt actions have extractors. .223 (rimless) autos have extractors. Single shot .38-55 Ballards (rimmed) have extractors. NONE of them headspace on the extractor! ..." In all these examples, there is something the cartridge rests against when it goes bang, a bolt face, a breech block, etc., and each of them have an extractor. I won't take space quoting each different headspace method for each different case type, but suffice it to say, none of them headspace on "... what the case really is resting against when it goes bang.", nor the extractor.
Headspace is measured FROM the head of the case to some point in the chamber that is intended to "confine" the cartridge and "fix" it for firing. Let's say the headspace "ledge" wasn't part of a straight-walled auto pistol case (the chamber would need a taper at theat point into the rifling) and that the extractor face was the headspace point. Now think for a minute. How critical would the extractor face measurement be, from the breech face? Would an occasional cartridge slipped off the extractor and be sent into the chamber? If you would rather depend on the extractor holding the cartridge against the breech face, then be careful about some recommendations to customize the extraction angles on the extractor face, because that will cause some relaxation of extractor tension in battery. I think the extractor does play a part in keeping the case against the breech face. It's just not how you measure headspace.
After I reload a .45 ACP case several times, the case rim becomes a little battered and misshapen. I throw out a few that are really beat up, but the variations there are pretty large.
There is something in WESHOOT2's post that I may have read my own meaning into. And on re-reading it more carefully I see another take. <BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>2) Auto cartridges headspace based on extractor tension holding the cartridge against the breech face.[/quote] That doesn't say that they headspace on the extractor, which I originally interpreted. But it does imply that the extractor keeps the cartridge from flying forward when chambering. Chris described that too. That much I agree with. But, sorry, that's not headspace.
I haven't read the American Handgunning article. I don't have a subscription. I'll look for it on the magazine racks.
As far as the .357 Sig goes, I'm gonna hang it out here and say I'll bet a dollar it headspaces on the shoulder. Most of the .40 brass used to make it would come out too short to headspace on the mouth. I don't know of a shouldered (bottle neck)round that doesn't headspace on the shoulder. Now I'll go to Chris's link and see if I owe you a dollar.
Happy headspacing!