From: Winnipeg, MB
I believe there was another similar battle where both the cruiser and the AMC were sunk.
Never mind, that is in fact the battle where that happened. Though the Sydney sank first, the Kormoran also sank.
Kormoran torpedoed the unwary Sydney and used her guns to smash the cruiser topside. I have not heard of any shells piercing Sydney's armour belt, but when you have torpedo holes below it, you don't really need to go through it.
Sydney was battling heavy fires, probably with limited ability to pump water thanks to the torpedo hit(s) knocking out engine power. The crew of Kormoran (in lifeboats) saw a huge explosion over the horizon where the burning Sydney was, suggesting the fires set off the CL's magazines or torpedo warheads.
So I rather doubt that AMCs carried AP shells - they would pass through the intended prey, unarmoured merchant vessels.
There is an episode of Drain the Oceans that covers this battle. Based on the damage found on the wrecks, the Sydney's bridge, and first two turrets were taken out by gunfire before the ship was torpedoed. The turrets appear to have been hit by AP rounds.
These sorts of programs aren't great on the details. For one they kept referring to the Germans as "Nazis". They said the guns on the Kormoran were modified AT guns. I don't know why the Germans would do that, they had a range of naval guns from S-boats on up to large battleships. They could have easily mounted some small caliber naval guns.
Just looked up Kormoran in Wikipedia. Main armament was 6x 15cm (5.9 inch) guns but it did have two 3.7cm Pak 36 a/t guns and 5x 20mm flak guns. It also had six TT with 24 torps. Her marine diesels could make up to 18 kts. speed.
I can only guess the a/t guns were for pinpoint targets like trying to hit the radio room of a ship so it did not send out a distress call, or maybe the bridge or defensive guns.
No matter how bad a situation is, you can always make it worse. - Chris Hadfield : An Astronaut's Guide To Life On Earth