Whish is why I scrap 9MM brass that fails my Wilson case gauge after sizing, and I agree, the plunk test is for OAL, although it works if the ammo is only ever fired in that one gun.Yet another reason to case gauge rounds. 9 major brass that has been heavily stressed (either because it is monstrously over pressure, or because it has been overloaded a few times) usually gets expanded at the base - where regular sizing dies cannot reach. That brass will then not pass a case gauge, even though it will easy pass a "plunk test," particularly in a barrel with generous clearances around the base like the OP's.
I continue to maintain that the punk test is for setting up COAL, and case gauging is a more exacting a reliable QC check on finished rounds.