Designing the SE Berm

The Berm, with a straight handle

After the Mesa, I wanted something simple and easily repeatable, as that would make it easier for others to reproduce it as well. To that end, I wanted to try to make it completely 3D printed, and not require any other tools to build.

Initial Design

I had some ideas for what I wanted. It definitely would have to be a pistol format, and I wanted a simple design. I had some ideas for a Welrod style blaster, so that's what I decided to focus my efforts into. While working on figuring out a mechanism for a magazine version, I decided it would be easier to just start out with a single shot variant. The Welrod style also helped with some design criteria for the blaster. True to its stealthy covert origins, I wanted my blaster to be quiet and inconspicuous. In addition, I gave myself the challenge of making it completely without any metal, which means no conventional springs or screws to hold it together.

The Welrod MkII, the inspiration for this build

To get around the no metal limitation, I decided to power it using a rubber band and direct contact with the dart. A plunger system would be too noisy, especially if the cylinder had to be 3D printed, and a rubber band is naturally pretty quiet when hit. For the trigger and catch, I am using a printed leaf spring to push on the catch, which also pushes the trigger forward. I had used printed leaf springs a few times in the past, and it seemed like a reasonable choice for this project. Like the Welrod, this wasn't meant to be a service pistol, and would be sparingly used, so plastic fatigue wasn't as big a deal. To hold everything together, I designed the body as a monocoque structure, with the front and back held in by the tension of the rubber band.

The monocoque body which encloses everything but the trigger

The CAD process was fairly straightforward for a simple blaster like this. I made sure to pay attention to print orientation, and design parts to minimize the need for supports. For example, the locking lugs for the front piece is printed at an angle, such that no supports would be needed when printing it out.

The angled lug, meant to mate on a triangular surface on the body to reduce supports

One of the things to take into consideration is the elongation limits of the elastic you are using. For example, I was originally looking at using shock cord for powering the blaster, but they typically have an elongation of 100%. That means that the cord will safely stretch 100%, or twice its original length. What this means for me is that the primed length of the elastic must be no more than twice the resting length, which forms a constraint on the length of the blaster. I could make the blaster longer so that it isn't a concern, but I wanted it to fit in most print beds and stay as compact as possible, so the trick was finding the shortest it could be, yet still stretch enough to fit a dart in.

Designing the catch mechanism was a little challenging. Originally, I researched on crossbow style catches, as they would have better trigger releases than any traditional Nerf catch I was used to. However, many of these designs either took up too much space, or required the use of torsion springs, which would be very difficult to print. Hence, I went for the tried and true sliding catch, which has its benefits and drawbacks. In the sliding catch, the band would get caught behind the catch, and the trigger would pull the catch down, which releases the band. This would require a single spring to keep the catch up, and the trigger could piggyback off that spring by using the angled surface.

Closeup of the catch. The catch is highlighted, and the mating surface and spring are clearly visible

It was fun putting little a few homages to the Welrod as well. For example, there are no identifiable markings on the blaster, not even a single logo. The knurling on the pull handle is a 1:10 twist, same as the rifling used for the 9mm Welrod. The front is slightly concave similar to the dished front of the actual gun, and even the diameter of the blaster (1.25") is exactly the same as the diameter of the real one. That was fortuitous, as a dart is considerably larger than the .32 and 9mm rounds used in the Welrod.

Prototyping

This blaster went through a few more iterations compared to the Mesa, primarily because it was much easier to make more of them because it was entirely 3D printed. I had a total of three bodies printed, and quite a few more smaller component variations after that.

The three prototype bodies

The first issue I ran into immediately was finding a suitable rubber band. It was difficult finding a rubber band in my assorted bag that had decent tension, yet could stretch all the way when it was primed. These issues really only became apparent after I got a tangible product in my hands.

The 3D print process also revealed some tolerance issues. My original body didn't have the angled locking lugs, and I found that the leftover support remnants were nearly impossible to remove because of their location, and interfered with the locking. In addition, when printing the pull, the arms were thin enough and tall enough to start wobbling when it got higher. This lead to a messy print about 3 inches up. I had to redesign it slightly to be wider and not wobble as much as it printed.

The immediately obvious issue I had with the first prototype was that there wasn't enough room for the catch to go down with the spring, and therefore it couldn't actually fire. You can see pretty clearly in the above picture that the trigger box under the cylinder on the first prototype is much shorter than the other two. That had to be expanded so that the spring doesn't impact the bottom cap, and doesn't restrict the catch.

The printed leaf spring actually performed pretty well in my testing. It did however start taking on a permanent bend after maybe 50 or so actuations. To get around any issues that this might cause, I made the spring reversible, so if it starts bending one way, you can simply flip the spring over and force it to bend the other way. This worked out pretty well, and didn't seem to cause any problems with plastic fatigue. I think other plastic materials might perform better here, such as ABS or PETG.

One gnawing problem that was manifest throughout the prototypes was that the trigger pull was incredibly stiff, and would not always release the catch. It took a lot of testing and troubleshooting to fix that, as there were multiple causes for that problem. The first cause, as mentioned above, was that there wasn't enough room for the catch to go down. After that was fixed, it was found that since the trigger was so short, it actually sometimes got caught on the lip of the body where the catch is. That corner had to be chamfered to smooth it out. After that, the trigger was overextending, and the flat part of the trigger ran into the angle, meaning it would not reset. This was fixed by increasing the thickness of the trigger, which also mitigated an issue of the trigger nub snapping when too much force is applied.

I originally designed it with a straight handle, as that was super easy to prototype. However, after holding it for a bit, I didn't really like the ergonomics of it. Thus, I made an angled grip that was much more ergonomic to point with, although it comes at the expense of uneven dart storage lengths. I wasn't sure what people would like more, so I left both as options. I ran out of filament towards the end of the build phase though, so I didn't print out the angled one.

Both handle variations, straight and angled

Conclusions

Overall this build succeeded in what it was designed for. It fires roughly the same power as a Jolt based on the rubber band I used, yet is 15dB quieter. This makes a huge difference from further away, and the sound signature is very different from the usual pop of the dart leaving the barrel. Instead, the loudest sound is actually the click of the trigger bottoming out, which can easily be mitigated by putting a bit of rubber padding behind the trigger.

Sound comparison one foot away from the microphone, with the max dB representing when it was fired

There were definitely some unexpected challenges during the process, particularly with the trigger and catch. There are many things that you just can't get from simply going through the motions in CAD, so that prototyping step is hugely important. There's a few changes I would like to make eventually to the mag fed variant based off of what I learned from this project. For one, I am aiming to use 1/8" shock cord for the full size variant, as the length makes it safer to use shock cord up to its elongation limit. The trigger mechanism will need to be redesigned, as there will be a magazine in its place. However, I do like how everything on the body is held together using just the tension of the elastic, and the monocoque construction is reasonably easy to work with and print.

Similar to the Mesa, the files for printing your own can be found on my Etsy. It's a fairly light print, taking up less than 90g of plastic to print, and 70g without the handle.