Here's something interesting I discovered a while ago. Those little pieces meant to hold shoulder armor that came in those spartan armor customizer sets can actually peg into a figure's arms. I made a couple robotic arms and plugged them into some Infinite marines but I found they work with CoD as well. You can see by the first picture they can actually shoulder a rifle decently though the right arm is a bit awkward for this as the shoulder loses its universal articulation and is now a swivel. Still, it looks pretty cool and I'm interested in how this idea can be built upon.