The simplest thing to do is use the rig for the machine-gun and parent yours to that. You'll need to inspect the D3 weapon in Blender and look to see how the mesh is vertex weighted to the Armature, then basically replicate whatever you find on yours. It should then be a simple matter of exporting the MD5.
Note that depending on how much work you want to do, once you're exported the mesh you'll probably want to rename the assets (mesh and textures) to match the name of the weapon your replacing, that way all you're doing is a mesh over-ride. Otherwise it means you have to make a completely customised set of def files which determine what the weapon does and what assets the game should be pulling in for use (fire/reload anims, textures/skin files etc.).
Make sure you optimise that mesh before you try using it, it looks like you have the sub-divide modifier assigned which will bloat the structure with a huge amount of unnecessary polygons.. "less is more", always keep that in mind.
