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I can see how the "O" ring effectively eliminates the headspace buy thrusting the cartridge firmly against the bolt face but how does that centre it as well?
If it's given there is headspace to start with, and it's also given that the diameter of the tapering chamber is larger than the tapering cartridge diameter, then I guess it means that by thrusting the cartridge against the bolt face and away from the chamber this would maximise the airspace around the cartridge.
If the cartridge is truly centred, (assuming the bolt face is perfectly square to the chamber) there would be a perfectly equal air gap around the cartridge and no contact at all with the chamber wall.
I would imagine that you need to remove the extractor for these first firings as I would think that it would push the cartridge away from itself.
The big question....
Where or how does the cartridge reference itself to the centre of the chamber ???
How is this achieved by the "O"ring??
Danny

You may overcome most of the extractor's influence by using a properly sized o-ring, but if you're really worried, then, yes, removing the extractor would be possibly beneficial- "possibly", because the breeching area is not uniformly surrounded... Plus, it's a boatload more work.

If perfectly done, no, there's no contact between cartridge and chamber, unless the bullet is seated out far enough to engage the rifling. Would it help shrink groups as well as extend brass life? Theoretically. In practice? Nobody's "fessed up" yet! (Or did I miss something?)

Benchresting Enfields is not high on my shooting exercises list- they seem to invite more "field" and "position shooting" usage!

ETA Sorry if I drifted off the intent of your question, Danhar1960. My pea brain got sidetracked on implications of the question of which you may not have intended. Besides Son's drawing below does the job far better- except, well, I'm still not sure that unless the ring is fairly small, there may not be some unintended non-concentric side loading as the ring "oozes" out the breech. Mr Horton's o-rings appear fairly large-ish.

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Originally Posted by Danhar1960

I can see how the "O" ring effectively eliminates the headspace buy thrusting the cartridge firmly against the bolt face but how does that centre it as well?
If it's given there is headspace to start with, and it's also given that the diameter of the tapering chamber is larger than the tapering cartridge diameter, then I guess it means that by thrusting the cartridge against the bolt face and away from the chamber this would maximise the airspace around the cartridge.
If the cartridge is truly centred, (assuming the bolt face is perfectly square to the chamber) there would be a perfectly equal air gap around the cartridge and no contact at all with the chamber wall.
I would imagine that you need to remove the extractor for these first firings as I would think that it would push the cartridge away from itself.
The big question....
Where or how does the cartridge reference itself to the centre of the chamber ???
How is this achieved by the "O"ring??
Danny

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Danny, I just knocked up a rough sketch in paint to show how the o ring will centre the base of the case in the back of the chamber.
The rubber o ring is uniform is section all the way around and flexible. It will be squeezed into the space evenly all the way around by the bolt being closed onto the case. This should centre the rear of the cartridge regardless of if the boltface is square to the chamber or not.
I think the o ring would be firm enough though to negate most of the effect of the extractor bearing on the right side. In fact, if you watch the extractor closely as you cam the bolt shut, you may see the extractor being pushed outward by the case being centred- either that or it's seating onto the machined slot on the barrel- taking the weight off the case.

I remove the extractor because it effects the centering of the case and the o-ring lasts longer and doesn't get small nicks or cuts from the extractor.

The case is centered in the chamber because you are compressing and flattening the o-ring. The same way a hydraulic piston is centered in the cylinder of a actuator under pressure. If you notice the rear of the chamber is beveled and as the o-ring is compressing it moves inward centering the case.

The o-ring has only two directions in can move when compressed and this makes it self centering.

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Originally Posted by Son

It will be squeezed into the space evenly all the way around by the bolt being closed onto the case.

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Thanks for taking the time with the pic and the info Son. I can see that it would work.
Really should think seriously about reloading my HXP as I will soon be under a 1000 rounds left and there isn't anymore around. (That I can find anyway)
Danny