Will say, this is the second article or post concerning putting a compressible spacer above the rim, to hold the case back against the bolt.

This will not work in belted, rebated, nor rimless cases. It might work OK with semi-rimmed cases.

Necking the case up then forming a new shoulder for a crush fit on bolt or action closure PRIOR to firing the first time, is a better method, imho.

I measured some various makes of .303 brass that were in a junk bowl on my bench. They ran from .055" to .064" on rim thickness - how about them apples?

By the numbers

1. The firing pin shoves the case forward, rim against the breech.
2. The primer detonates. If it's not heavily crimped in place, it backs out, shoving the bolt and barrel as far apart as it can.
3. The thin, forward part of the case expands to fill and grip the chamber while the bullet moves out of the case and down the barrel.
4. The solid case head can't expand and grip the chamber, so it moves rearward, re-seating the primer, stretching the case walls just forward of the head, and stopping when it hits the bolt face. (In rear-locking actions like the Lee, the bolt and receiver also compress/stretch to add a little more movement. The higher the pressure, the more they move.)
5. If (and only if) the amount of head movement exceeds the elastic limits of the case, the cartridge separates into two pieces.

New cartridge cases can normally stretch a lot before breaking. Even with a minimum rim .054" thick and maximum "field" headspace of .074", the resultant .020" head clearance is well within the limits of new brass and it's very unlikely a new case will separate even if the headspace is somewhat more than the field maximum (which is pretty rare).