Editorial: The Liberator – First 3D Printed Gun – Is Stupid From an Engineering Perspective

Posted on Posted in Eng/Sci/Tech, Opinions, Projects, Reviews

The technology is here, and our politicians are trembling. But mature it is not.

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Cody Williams and the rest of the Defense Distributed gang have just recently unveiled the world’s first fully (94%) 3D-printed gun, and boy is it ugly. Although it is understandably just a token milestone in their long-term goal of achieving gun ownership for anyone who wants one, the Liberator’s design appears to be a brute-force amalgamation of “whatever it takes to work.”

Wilson (a law student) admitted in a news article that he set an ambitious deadline for himself, but as an engineer I can’t help but pick up on the numerous ways this design could be improved. Let’s start from an earlier point in Defense Distributed’s* history. (*from hereon called DD for short)

AR15Lower

One of the first milestones to send shockwaves through the gun-related communities was the unveiling of DD’s 3D-printed lower receiver for the AR-15 platform. This is the component that’s heavily regulated because it contains the trigger and other essential components that make a gun deadly. Without it, you would have nothing more than a high-tech musket.

But Wilson wasn’t thinking like an engineer. He was thinking symbolically. He wanted to make an established firearm with 3D-printing techniques. The first models failed rapidly under testing, and in a video interview he admitted that he went through numerous design revisions adding material and reinforcement to structurally weak areas of the receiver. He was doing it all wrong.

You cannot expect 3D-printed materials to endure the same stress loads as metal components. Guns aren’t built for safety-factors like aerospace engineering projects, they’re built for ergonomics. If you look at the lower reciever pictured above, the right side is where a stock is screwed on. It’s a natural stress concentrator, exacerbated by the fact that the interface between the stock and the lower receiver is based on threads. But it works because guns are made from metal. Pinned connections that hold gun parts also work because the metal thicknesses used in guns can take the shock-loading of gunfire. With a 3D-printed lower, these small holes are fracture points waiting to happen.

The correct way to deal with this is not to just double up the thickness of parts, but to understand that the joints in traditional firearms are inherently BAD for 3D-printing techniques. That relatively thin neck on the receiver (where the webbing between your index finger and thumb would go) that leads to the stock threads should be thicker and have a gentler curve. The threaded section, assuming you keep it, should be longer so that the force can be distributed over more threads/teeth.

The ideal solution would be to just print the stock directly attached to the lower receiver. And before you ask why the stock normally screws on like that, there’s a buffer spring that has to pass through the back of the lower receiver (don’t worry, I was a noob too and didn’t realize at first).

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Let’s move on to the Liberator. Where does this design fail? In many places. 3D printed materials are generally all some form of plastic, which have a limited variety of mechanical properties. Trying to make spring-like devices from stereolithography is inefficient at best. And there are plenty of non-metal components that could replace Wilson’s parts quite elegantly. Taking a cue from Jörg Sprave, rubber can be used to make a very effective, repetitive motion or application of force to move something like a firing pin.

Furthermore, joints that connect things like pistol grips to the receiver are unfortunate but necessary stress concentration points. As much as it would be cool to have a wrist-mounted gun (Assassins Creed-style), it just wouldn’t work from a precision point of view because human arms are awkward to place parallel to the line of sight to your target.

Instead, Cody Williams should take a cue from the IWI Tavor sales pitch about maximizing your points of contact with the weapon. A foregrip or stock would go a long way towards reducing the stress at the grip of the Liberator (although it would make it less of a pistol admittedly).

And yes, I know I’m missing the point: “It’s a 3D printed gun for crying out loud!” Yes it is, nicely done, mission accomplished. But my point is that 3D-printing is not a silver bullet for revolutionizing home-manufacturing. Lego doesn’t *only* build bricks, they have other specialized parts because there is no one-size/formfactor-fits-all solution to every build.

Thoughtful engineering is more important than ever now that fabricating abstract geometries is as simple as “point and click.” The Liberator is a cool (and thought-provoking) concept, but it still has a looong way to go. And there will almost definitely be “some assembly required.”

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