The o-ring method of fire forming case to extend their life

Saying that 6.5 Jap chambers are "generous", is generous in itself.

Whilst they are not even remotely "Bench-Rest" standard, they worked pretty well for "government work".

If you are prepared to do a bit of fiddling with reloading dies and start with GOOD brass (Norma springs to mind, IF you can find it), you will get "reasonable" life from your cases.

All of which is slightly academic. The original users were not exactly obsessive about recovering their brass on the battlefield and then studiously reloading it. The cartridge case did its job of keeping the powder dry and the components together in a handy, weather-resistant package until the moment of discharge, after which it was usually flung unceremoniously into the wild, blue, (green, or brown), yonder.

This applies to pretty much ALL service ammo in the last century or so, with only a few countries "economizing" by reloading their cases, usually just ONCE.

ANY ordnance system using Mercuric-based primers would be running a serious risk of a LOT of case failures on second or subsequent reloads, or even just cracking "in the packet", just as Lee Enfield "enthusiasts" have endured for decades.

Thanks to everyone who have responded. However, I am going to have to read through these again, as I am somewhat confused. My understanding from what I had read was that this was a good idea to extend the life of the case, with subsequent neck sizing only between reloads (using the same rifle of course). I had assumed that I was just compressing the rubber and I don't have access to the necessary gauges. However, my understanding was that this was less of a consideration.

Mayhem, you have it right. Use the O-rings, expect the extra resistance as you close the bolt, subsequently only neck size. It all helps, you're on the right track.

Interesting experiment with the "rimless" .303.

I saw a couple built on P-14 rifles and using M-17 bolts and some magazine components, many years ago. The sages at the time of this "fashion" reckoned the old SMLE was not up to the task of stiff loads in this modified cartridge; more likely they would simply not feed from the magazine reliably nor extract consistently because of the altered case geometry.

History repeats.

Back in the 1950s, here in Oz, there was an abundance of .303 ammo AND "souvenired" rifles from all sorts of interesting places.

A moderately popular conversion was done to Jap. Type 99 7.7 rifles which, conveniently, use the same (roughly) bore dimensions as .303.

Two approaches:

1. Open out the bolt face for the .303 rim, sleeve the chamber and cut a new .303 shaped chamber. Crude, and feed from the mag was "marginal".

2. Get a bit "Gucci" and turn the case heads down to "rimless" dimensions to more-or-less match the 7.7 bolt face and sleeve / recut the chamber as before.

Had an uncle with a specimen of version 1, even fired it with a bunch of FN .303 ball ammo. Mostly functional, but a bit long in its original trim to be a handy-dandy "farm gun", which is what he used it for. I guess he reckoned that, having relieved some unfortunate Japanese soldier of his burden, he may as well tuck it in his big box of military engineering equipment, along with sundry bayonets and a couple of 6.5 Jap carbines, and see if he could get his "momentos" home: he did.

I heartily concur that the relatively tiny .303 shoulder is adequate for "headspacing". The problems arise because of "variations", (some of them "after-market" / "custom"), in the chamber dimensions.

If you can arrange for your cases to "fire-form" ONCE to a specific chamber and to then NOT over-work them by full-length sizing back to CIP / SAAMI / "Mil" spec, you will extract greater life from the increasingly expensive brass.

Of course, this approach may mean having distinctly separate ammo for each of several .303 rifles. Not a huge challenge for the organized person.

Mayhem... There are other considerations to keep in mind. Yes, the o ring method will centralise your cases in the chamber so they will blow out evenly all round, and then, with the cases headspacing on the shoulder and neck sizing only, they will last longer. Well on the way to maximise brass life.
Bear in mind the earlier comments about brass flow. If your loads are anything above mild, your cases will lengthen with firing. You must trim to length every reload. They can lengthen into the leade and crimp the projectile on chambering, causing increases in pressure when firing.
The other thing is the cases will still stretch and eventually (i found two neck sizings on MkVII equivalent loads to be normal) the cases will have resistance when closing the bolt. Then you need to have an adjustable FLR die to set a thou or three shorter to relieve the load on the locking lugs during closing. Then you can neck size and length trim for another two or so before bumping the shoulders back a minimal amount again.

I have cases that have done 7 or 8 reloads to MkVII spec without any signs of failure.

Thanks to all that have responded. I will certainly be checking the length of the cases between loads.