...when the firing pin smacks the primer, that drives the case forward hard into the chamber shoulder centering the round very well in the chamber up front. There's no "rattle space with either one; both are held very repeatable in centering up front.
I'd be in your camp (at least thoretically) were it not for one nasty piece of empirical experience back in `85.
I'd gotten/been given my father-in-law's captured Type 38/6.5Jap that year, and immediately gotten some Norma (¿best-there-is?) brass to load up. Bang! went the rifle; thwock! went the bullet in the cardboard backer; and Wow! went the shooter when he looked at the case's lower half when ejecting.
"bulge" is incorrect spelling.
" is the corrected term.
As in many a physics experiment, sometimes the subtitles get lost in fine-grained data -- so you increase the differences until you get obvious 'boundary value' results. In this case the oversize Jap chamber/undersized Norma case was obviously pushed to one side of the walls by the extractor even after firing pin strike. Even moderate pressure gas expanded to fill (chamber) space available ...and the rest is history.
Bottom Line 1: I finally went with 243Win cases to fill the chamber and things began to behave themselves.
Bottom Line 2: Extractors -- especially big hulking Beautiful Mauser claws -- will definitively push (and keep) overly-sized cases to one side of the chamber during ignition.