If it's not a heavy machine gun, or a light machine gun, it's a submachine gun. Caliber? Pah. PDW stands for Personal Defense Weapon. Basically any submachine gun could pass for a "personal defense weapon". I'm pretty sure about that.
EDIT: I wiki-owned myself
I really don't see why it would be that way, though. It's illogical.
On the other hand, the FN P90's wiki says it's a submachine gun. I guess the definition of both terms is up for some debate.
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