Jimro, in-line ejectors in the bolt push the loaded round forward until its shoulder stops centered in the chamber shoulder. If that don't work the smack of the firing pin does it. So the rimless bottleneck round's well centered up front where it counts when it's fired. Most extractors push the back of the case against the chamber wall so there's clearance opposite that point.
Take several primed empty cases, blacken their shoulders with a marker, then chamber them and pop their primers. Then look at the marks all the way around them from their impact as the center in the chamber shoulder. Use a case headspace guage to measure the cases both before and after firing. See how much the shoulder's set back from firing pin impact. That happens before the primer fires. How much depends on firing pin strength, pin protrusion from bolt face, primer cup hardness, case & chamber shoulder area and angle as well as the friction between the case brass and chamber steel.
Regarding a chambered round with no extractor or ejector putting pressure anywhere on the case, the case head could be anywhere when it fires. When the firing pin smacks the primer, it's possible that an off-center strike could push the case head sideways as the dimple was off center on the primer's anvil. And the way the case behaves as it's shoulder drives hard into the chamber shoulder may also move the back end of the case laterally. To say nothing of how the bolt head may center relative to the chamber when it's closed. The back end of a round so chambered is free to move with whatever other forces get applied to the case before the primer fires. I doubt all the rounds' back end is at the same place for all shots fired as long as the pressure ring's diameter is smaller than the chamber at that point when the round's chambered.
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Last edited by Bart B.; December 30, 2012 at 08:00 AM.