Okay, okay I re-read the post and you guys are right. Using the shoulder in a rimmed or belted case is a good idea if there is excessive headspace in where it is supposed to headspace against. In a weak shouldered case this still wont get you good case life, but better than without eg

303 with excessive headspace on rim - 1 or 2 reloads
303 with excessive headspace on rim but using shoulder for headspace point - 7 or 8 reloads
303 with correct headspace on rim - 15 to 20 reloads
303 with correct headspace on rim and using shoulder for headspace point - no difference to previous

Perrys rifle sounds like it has tight headspace on the rim and should get 15 to 20 reloads (guess)

If I'm right someone can buy me a beer, if I'm wrong I'll probably have one anyway