I'm guessing that like the .308 in the bolt gun I saw, Hatcher's gun eventually headspaced on the extractor hook as he reamed the chamber progressively deeper for each test, and that set the limit to how far back the head had to back up to stretch the pressure ring out. Once he got the chamber long enough to hit that limit, all further case expansion was, as you suggest, the shoulder blowing forward by drawing on neck brass. Still, its good to know extractor headspacing is tight enough to prevent a separation with new brass.
I found
this article by a fellow who accidentally fired some .308 in his .30-06 Garand at the National Matches in 1964. If you scroll down it includes a photo of the case. The self-loaders, of course, don't let you feel whether the case is going in a little on the firm side or not.
Nick