Here's my first step in turning Aggressors into terminator looking guys for my Salamanders.

I've uploaded the .blend file o you can try posing the hand yourselves if you want something different.

First time I make something this complicated and try to rig so it's a bit of a mess:

Start posing it from the shoulder. All parts are parented to the ones before them after that

Most of the parts have their rotation locked so they can only move in the direction they are supossed to, others, like the main body of the fist, need to be rotated with the local frame of reference and locking the rotation to the Y axis.
The power cable is controlled by the empties and has to be adjusted manually if the fist is rotated.

I've scaled this with a MkII torso that is suppossed to be in scale but you should probably check the measurements just in case.

This is the base model without detail, look at my profile for detailed variants and other weapons

Edit: I fixed the mesh on the elbow join

Edit: It's recently come to my attention that since I modeled these after the tartaros terminator arms with aggressors in mind the size of the shoulder piece is a bit too big to really fit a normal marine. If you want to use it for them I suggest loading the stl on blender or similar and scaling the shoulder piece down a bit with the very top as the reference point.