Actually, what I started with was a bunch of generalised parts. Narol gave me a bunch of weapon ANIs to look at and I noticed that most of the volition weapons are built using a building block approach. There's stock parts that are customized for each model.
I'm doing that, plus careful booleans (thank goodness LW booleans WORK cleanly) and some... er cheating.
The cheating involves looking throught the stock models that came on my Lightwave CD and using bits from them. The multibarrel .50 cal I'm working on is built along the same lines as the machine gun from the Hummer model, for example. In other cases, I took the face of the calculator and the keyboard from the notebook computer and messed with them a bit to make them suitable for use as detailing.
I can't express how good it feels to not have to worry about polycount for a while.

Does anyone else do this sort of stuff?