Back in the 60's the head stamp DEN 42 was an AP round and known for being corrosive. Also very accurate. The ammo was plentiful and cheap and often made available for match shooting at the club I belonged to at the time. Corrective measure at that time was to remove the action from the stock and run real hot water through the bore. I had a small hose that I inserted directly into the chamber for about 15 or 20 minutes. This heated the barrel hot enough that it pretty much dried by itself. Then as a final step, ran a oil rag through the barrel several times.
Peace is that brief glorious moment in history when everybody stands around reloading.
Author unkown.