"Mission kills" versus "Platform kills." A lightweight torp, a single, is supposedly okay to mission kill - i.e. take away a target sub's ability to perform effectively - most any SSN.
A platform kill would take probably take multiple hits. Someone once told me that NATO estimates it would take around a dozen Mk.46/54 to platform kill an Oscar II. I don't know how full of crap that guy might be, but seeing as how the warhead is just about equivalent to 300lbs of TNT (same as a WWII depth charge), I can see it.
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