As I remember it is in the designation, SmK standing for (checks sources) 'Spitzgeschoss mit Kern' (translated as 'Pointed bullet with core), R for Reizstoff (meaning 'irritant', it had a pellet of tear gas in side that was supposed to help knock out the tank crew, unfortunatly the pellet was so small that the allies had no idea it was there until they captured some of the ammo) and Lspur for Lechtspur ('light tracer'). I suspect that the round was unique because of the 'Reizstoff', if I am allowed a second guess then I would say it was the core which was hardened steel, a lot of sources claim it was tungsten carbide but this seems to be incorrect as tungsten was in short supply and not used for this type of ammunition.

So, am I right?

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There's no honorable way to kill, no gentle way to destroy. There is nothing good in war. Except its ending.