I often shop for used brass for my reloading and just recently got an order of four different rifle calibers from a brass dealer. For some reason, he included a couple of .303's in the mix. They showed why I don't even bother looking for once-fired .303 brass, all had a very pronounced swelling just ahead of the web. Apparently a lot of these rifles have very large chambers for functioning in extreme conditions but brass fired in those chambers is forever stretched beyond usability.
I've yet to reload for the .303 but I've heard of a procedure for the first firing that should eliminate the excess stretch. Find a rubber "O" ring that fits snugly around the head of the case before you fire that first load. The rubber holds the case firmly back against the bolt face as it is fired so the shoulder fire-forms to the chamber, hence forth those cases so treated fit that chamber and can greatly extend their reloading life.
So, that is the theory, has anyone used this process? And if you have, where did you find the appropiate o ring to use?