If you don't define something, then people complain that there isn't any definition and therefore determinations are arbitrary.
If you define it comprehensively, with ten or twenty degrees differentiating different levels of egregiousness, then people complain about excessive bureaucracy, and complain when even the twenty degrees aren't enough to differentiate two things that in their own minds should be considered differently.
My personal preference would be to hinge the distinction on potential number of people harmed. It shouldn't be hard to come up with an objective way of determining what can harm whom - after all, the Mythbusters can very effectively do so.
I would want any firearm with a larger clip to be classified as a WMD. The medium of dispensing harm shouldn't matter.