"Vici verci"? On this topic, as they say, (in a Marlene Dietrich accent), in show-biz, "I'll show you where the "Vice is Versa"".

Headspace is important for assessing the potential "state of play" regarding striker "reach" and the possibility of primers backing out.

BUT, the latter is not a big problem when the "approved" ammo has its primers firmly held in via a heavy annular crimp.

As long as the striker has enough "reach" and power behind it, things will work. How many folk obsessively gauge their striker protrusion AND striker spring pressure?

As for "case life": in the "real world" of "authorized" ammo supplied, via tax-payer Dollars, Lira, Roubles, etc., the stuff is generally on a one-way trip after issue to the GI, Digger, Ivan, etc.

All through its production and even in storage, ammo itself is subject to its own rigorous regime of testing and gauging.

On the "two-way" rifle range, NOBODY is likely to be ferreting around trying to recover their brass during a "hot" contact or "fire and movement" operation.

For the rest of us, a "sloppy" chamber will radically shorten case life, as the brass is blown outwards to fit said "dubious" dimensions, and squeezed back via reloading dies. Millions of Arisakas operated just fine with "out-of-spec" chambers. They were "in-spec" just where they needed to be for reliable ignition and extraction, ONCE per cartridge.

As for actual specs; a quick comparison of "SAAMI" and "original" .303 chamber drawings reveals remarkable similarities. What happened in the "real world" was another thing altogether, ESPECIALLY after the rifles left the tender care of PROPER Armourers and Ordnance techs.