Mil Tech — Army Research Laboratory Developing “Third Arm” for Soldiers
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Alan M. Petrillo is a Tucson, Ariz., journalist who writes for national and regional magazines and newspapers. He's the author of several books on historical military firearms; two historical mysteries, Full Moon and Asylum Lane; and his latest historical thriller, A Case of Dom Perignon; all available at

Photo courtesy of Army Research Laboratory

The U.S. Army Research Laboratory (ARL) is developing a device characterized as a "third arm" that can be attached to a soldier's protective vest and hold the soldier's weapon, which frees up his hands for other tasks.

"We designed the third arm to be able to interface with nearly any tactical plate-carrying vest," says Daniel Baechle, ARL mechanical engineer. "We created a lightweight carbon fiber plate that slides inside and conforms to an armor plate pocket, and the third arm attaches to this carbon fiber plate."

Baechle notes the third arm can be attached at either the front or the back of the protective vest, on either the left or right side.

"If it's attached at the back of the vest," he says, "it allows the soldier to go into a prone shooting position."

Zachary Wingard, ARL mechanical engineer, adds the project began because the Army is looking at more powerful weapons for soldiers in order to increase lethality.

"Soldiers in close-quarters engagement have the same probability of hitting the enemy as the enemy does in hitting them, so there is no overmatch on the part of our soldiers," Wingard says. "So if there's a way to increase the potency of a weapon, which usually increases length, weight, and recoil, those elements make a higher burden on the soldier, lower mobility, and increase the soldier's metabolic cost."

Wingard says soldiers currently are overburdened, carrying 90 to 100 pounds of combat load.

"We've found that in terms of injuries, 30 percent of them are load-bearing injuries in the field, such as ankle sprains and disc ruptures, he says.

Wingard says ARL put together a team of specialists in biomechanics, psychologists, and mechanical engineers to develop the new device.

Baechle says the third arm is in the research phase, being interfaced with the M4 carbine and AR-16 type rifles.

"We're not advocating it should be used with current configurations of weapons," he notes, "but are trying to learn the ballistic characteristics of the third arm. In the future, we might have a weapon with no stock, where the recoil and load is redistributed to a third arm so the soldier has more control ballistically. Our hope is to have a passively stabilized weapon the soldier can operate with one hand, leaving the other hand free to do something else."

Baechle says ARL has done live-fire trials with an active duty soldier holding the weapon with one hand by gripping it at the pistol grip, where the soldier was able to easily hit a target, while his second arm was free. For the trials, ARL engineers attached a Picatinny rail on top of the weapon, which is where the third arm attaches to the weapon.

"In a future iteration, we want it to be quickly detachable from the body, and to be able to be quickly attached to a weapon," he says.

He adds that ARL also attached a 27-pound M240B to a third arm and found the device took 100 percent of the weight in a usability demonstration. 

Wingard points out the current version of the third arm, which is made out of carbon fiber, weighs less than 4 pounds, but ARL is seeking to reduce that weight to around 3 pounds, if possible.

Baechle notes the most important considerations of the third arm revolve around two main points.

"Does it improve marksmanship? Does it reduce soldier fatigue? We're now doing static live-fire shooting trials to answer those basic questions,” he says. “We also want to look at situations where the third arm could be used to shoot on the move and other scenarios where the probability of a hit is much lower and the soldier could benefit more from extra stability."

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