I think requiring 15 damage is too high. Very few melee weapons can do that much in one hit.
Here is complete list: http://pastebin.com/yPY7imn1
But if we toss aside things like energy axes and cult blades and subtypes of pickaxes, it boils down to:

the butcher's Cleaver
the surgical drill
the circular saw
the pickaxe (and all its subtypes)

No weapons aside from those are would be able to cause internal wounds.