What's more dangerous than a two-armed soldier? A three-armed soldier, of course.

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.

Third Arm is lightweight (less than four pounds) and doesn't require a power source.

The Army has tested previous versions of Third Arm, but this new one allows soldiers to use it while lying on their stomach. One sergeant was able to sprint and then dive into a prone position while wearing the arm with an M4 rifle attached -- something you can see in the video above.

"Right now it's a prototype device," Baechle says. "It's been getting a lot of interest higher up in the Army...we're using some of the interest to help motivate further development of the device."

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