Frederick, it's my understanding that if the cases show runout it just means they were not sitting in the centre of the chamber when fired.(relatively speaking, the chamber is much bigger than the case! They all are!) Then no runout after being fired with the o ring will not show if the chamber is concentric to the bore or not, just that the rear of the chamber that the o ring centred it on is concentric with the chamber.

Do a quick test. Pull a projectile out of a NEW round without stretching the neck, or better still, piece of commercial unfired brass. Drop the case in the chamber and see how much it rattles around in there!

Headspace (for a rimmed case) is the distance between the rear of the chamber and the face of the bolt head. For the bolt to close, that space has to be greater than the rim thickness of the case. The case will sit in the bottom of the available space because it is the bottom edge of the chamber that is lifting the case on the face of the bolt against the resistance of going up under the extractor claw as it is chambered.
When the primer is struck, the striker "nails" the case in the position it was sitting, and the expansion of the case wall out to the chamber is restricted on the bottom because of the striker holding it. Have a good look at the primer indent. Often you will see there is a tiny extruded ridge on one side that will be opposite to the side which the case wall expanded. This shows the rear of the case tried to rise on the bolt face to allow the even expansion of the case walls to centre the case. The striker indent will be slightly off centre, and the ridge will be toward the centre.
I will see if I can successfully photograph this and add it here.

I have written all the above purely from my own experiences and study of the subject. I'm not saying I haven't missed some other relevant possibilities, so I am very welcoming of discussion on anything here.

Brad.