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July 08, 2014

Have I Printed a Gun Yet?

To be honest, I'm really quite surprised at how many people have asked me this since I got my 3D printer. Our morbid interest with guns aside, I'm really surprised at the fact people think it's possible.

Yes there is world press that says it's been done, but not on a home printer. The problem with home printed guns... well, let's start with a picture - save me a thousand words.


See all that fiery stuff, well that doesn't play nice with plastic.

You then have physics. Newtons third law for a start, but more importantly the physics of explosions. The pressure build up in a bullet casing is huge and it has to come out somewhere - normally a rather thick metal barrel (ok, some ceramic and plastic barrels are out there, but the point remains) directing the explosive gasses in one direction to accelerate the bullet. In the case of a revolver (as the picture above) the gasses fly out the sides as well so less energy is used to accelerate the bullet.

With plastic you extrude from a nozzle that's only really good at temperatures under 50 degrees centigrade, the pressure in a printed gun barrel would rip it apart. The only "direction" the bullet would get would be the 1 or 2 millimeters of brass that holds the bullet in the cartridge. So, basically, you have a mini explosion in a plastic pipe bomb happening just in front of your hand, and while a bullet may move forward slightly, you will probably be worse off with all the molten plastic, gas and gun powder residue flying back at you.

The only real option is a tiny .22 caliber bullet - you'd probably do more damage with an air pistol.

Note however, I didn't say that I hadn't printed a gun yet. Roll out the rubber band gun. Cue the "A-Team" music.

I've seen a few online and in toy stores but I wanted to have another project to extend my design and build skills, so I thought about this carefully.
  • The easiest option would be a single shot, but I wanted a multiple shot semi automatic option - you can't keep stopping to reload during a zombie apocalypse.
  • It needed to be quick to load - when you do get a few seconds to yourself, every second counts.
  • It needed to be light, yet strong - carry more-a-bility.
  • It needed to be reliable - no accidental pre-release, every shot needs to count and beside, automatic weapons are illegal in the UK - even during a zombie apocalypse.
  • It needed to not require any 'extra' parts eg, screws, pins etc.
  • Ideally, this would print in one go - all the parts together, and between 3 and 5 hours.
  • I gave myself 24 hours to complete the design and get to the first usable item.

The Design

Looking at other guns on the web the basic operation was simple. a bit of a frame, a barrel type thing and some workings.

Lets start with the workings. Another, longer term, project I'm working on is a 3D printed clock so the release wheel and trigger control seemed to be like the pendulum escapement action on the seconds hands of a clock. Since I wanted a six-shooter, six spokes, and the appropriate escapement seemed to work. What size though, well, I thought about the forces and went for a pretty thick spindle, then doubled that for the inner dimension of the wheel, then added a little bit more than the rubber band thickness I had to hand for the total size. Rounding off the edges for a smooth release and the wheel was done, the related escapement was pretty easy based on my clock for the trigger and we were nearly there.

The problem with a clock though is that the travel is 50:50 per tick. This means that when the pendulum reaches one side it allows the wheel to turn but by the same amount as when it reaches the other. I needed my mechanism to release more to allow the band to fire. I tried a couple of things but basically with the trigger where it was and the directions of spin, I needed a slightly more complicated cantilever mechanism to do this. I therefore went for a simpler design that allowed for a nice easy loading action, but didn't fire the band until you released the trigger - no real biggy. A simple rig later and I had the basic design and sizes done.


That seemed like the complicated bit over with. I knew where 2 of the pins in the frame had to go. How hard can it be to make a frame... Bloody hard as it turned out. On the plus side I had already planned in some spacers around the release wheel for the rubber bands to wrap around the spindle, and I had built the spindles quite thick for support reasons. and I had left some more space to be able to replace the trigger spring while the gun was assembled. The first attempt on the frame was.. well. A bit big.


That took two and a half hours to print. Way longer than I wanted, and it was really too big to allow two to be printed together. The next attempt was... well, too small.


Kind of reminded me of Men In Black. I was also hitting the limits of the print bed with the built in barrel so I thought I'd do the barrel separately. Finally, in true goldilocks tradition, the third design was just right.


Oh, yes, the barrel is "on wonky" - thats because I needed to reduce the angle between the release point and the release wheel so the bands don't snag or deflect off the barrel as it released.

First Firing

Well, the thing worked perfectly first time. I was very surprised. All the thought I'd put into the tolerances for the frame pins meant the frame fitted together very well without any glue and the tolerances I'd built into the release wheel and trigger pivot were also perfect and loose enough without wobbling.

I had some #32 lazzie bands to hand (80mm x 3mm) and loaded that bad boy up. Here I noticed the gap for the release wheel was probably not wide enough for the #32's so I'd probably have to get some #18's, but maybe shorter ones may work better storing more energy... but then weighing less... Could I widen the frame... but it fits nicely in the hand. Also the last couple of bands flexed the frame a little between the trigger and release wheel... I should add a support strut in there.

While I was tweaking, I thought about the barrel. There was nothing wrong there, but it was quite wide and I'd like it a little thinner so the bands were stretched wider at the release wheel. Also, I could get the print time back under 4 hours if I thinned it a bit. The print time went up because of the strut I added for supporting the release wheel.

I did think about adding a pin just in front of the trigger and maybe one on the back, but there is no structural load here and this would only really increase the print time. I also pondered whether the back of the trigger sticking out would be a problem and decided it wasn't for what I was after.

The First Usable Unit

I had made it in 22 hours - 2 hours under budget. I still had to wait for the smaller ordered elastic bands, but it worked with the bigger ones. Total print time was 3 hours and 50 minutes with suitable internal support. The rubber bands I had shoot over 12 feet and are pretty accurate. Here she is in manly blue.


I am also an equal opportunities weapons maker so I also made one for the ladies so they could be helping out during that zombie apocalypse.


Talking of zombie apocalypse, I printed out some zombies as well.


Future plans

There are a few tweaks I'd make to this design:

  • I'd probably spend some time making the cantilever trigger work so the bands fired on the pull action not the release, also, more teeth would hold more bands without over-winding. I would need to take into account the forces in the multiple rubber bands though.
  • I'd probably add move the trigger mechanism into the frame a bit more. When loading the last few bands it's a bit hard to rotate the loading wheel and you can catch the trigger mechanism, stopping it loading.
  • I could rotate the trigger and release wheel within the design, this would allow me to have the best of both worlds with the release on pull and easy load, however I'd need a longer barrel mount and I can only just print this all in one go as it is.
  • I'd also probably invest in a safety mechanism, even a trigger guard would do as a tester 'accidentally' shot me in the head when they picked it up.

Also, I want to make one of these: