Worldcraft dude: You'd have to edit sequences so that both fires, edit the ammo amount, coding .
There IS NO EASY WAY of modeling.
I tried, then my Ms3d broke down.
Black Mesa: Milkshape is the only one (i know of) that can create Half-life models(or 3dmax), and 25$ is NOT expensive.
The ones who made Ms3d, what should they live off?
Added:
There's no way of copy+paste modelling