The post on the back of the shoulder joint ball should be cut off. It's just there to make the back of the shoulder ball print less awfully.
Anyway, print out the barrel, the front and one of the base plates. You'll need to drill holes and add wires.
I designed it by realizing I needed more rokkit launchers for my Killa Kans. Then I started reminiscing about Robotech, then I blacked out and here we are.