https://www.cnet.com/news/us-army-researchers-think-3-armed-super-soldiers-are-the-future-of-combat/

Engineers at the US Army Research Laboratory are developing an exoskeleton with a third arm for use in military combat. The exoskeleton, called Third Arm, helps distribute the weight of heavy weapons and can stabilize weapons for more accurate shots. 

In the video, ARL mechanical engineer Dan Baechle says that the arm was able to carry the full weight of a 27-pound M240B machine gun, making it easier for a soldier to hold in his [human] arms.