A 303 case extractor may work... but my feelings are it won't get up into the throat area to get that ring.  As I said before, get a piece of wooden dowl rod that is smaller than the bore.  Try to maneuver it to catch the lip and tap it out with a mallet.  BE CAREFUL to NOT strike the barrel/muzzle with a hammer when it breaks free!  If that doesn't work, then take it to a gunsmith.