I've noticed with the starter submachine gun (or whatever it is) I have a very tricky time severing limbs, as opposed to the old fashioned Plasma Cutter. I recall the the bullet based gun taking down human opponents well enough in the first chapter, so I suspect bullets are good against humans (and for head-shots) and the classic "tool" weapons like the Cutter and Line Gun are good for Necromorphs which need their limbs to be severed.
So are "bullet" weapons only suited for human targets? If so, are there exceptions? For instance I suspect a shotgun to be fairly decent at removing limbs, and the Pulse Rifle from the first game wasn't too bad at removing limbs either.