Headspace...

[Bruce_in_Oz]

Pondering further; how many folk have used and maybe still have, "rim-thickness" gauges for .303 ammo.

Back in "the day", no "well-equipped" full-bore shooter would be without one of these.

The idea, and it has SOME merit, is that different rim thicknesses, coupled with "correct" headspace, could produce ignition variations, and thus, different points of impact.

Hard-core competition RIMFIRE types STILL have their own versions of this gauge. These are used to select cartridges having a "rim" thickness" in a fairly narrow range, to reduce variations in the ignition of the priming compound contained within that rim.

It's mostly about thinning the herd of variables, not to mention a bit of "Zen" or, "The Force" or what have you. "Fashion statements" also have a role to play; see; "gadget gamesmanship".

A "THIN" rim will allow the round to "float' a bit, possibly lessening the depth and "speed" of the striker fall, and thus, minutely alter primer performance. Hard-core bench-rest types batch their brass, and gauge the seating depth of their "special" primers, after "uniforming" the pockets and the flash-holes.

Have a close look at the operation of the extractor on a Lee-Enfield: It actually holds the case fairly close to the bolt-face whilst in operation. Going back to J. M. Browning's 1911, the "as designed" chamber had NO distinct "step" where the case mouth could engage, just a taper (leade). The "floating" striker CAN protrude a LONG way forward, but it is not exactly propelled by a Mauser or Lee-Enfield grade striker spring. The extractor is expected to stop the relatively low pressure .45ACP case from "getting away". Browning hedged his bets with the .38 Auto / .38 Super, giving it a minuscule "semi" rim that is supposed to engage a "cutout" in the barrel hood. Not much point, as it turned out. Guns in the slightly shorter and MARGINALLY tapered 9mm para work just fine as well, without much of a chamber mouth "step".

Open-bolt SMGs are a different bucket of bait.