Designing a Homemade Nerf Blaster From Scratch - The SE Mesa

I've been modifying Nerf blasters for close to ten years now, but I've never designed my own before. I had been wanting to build my own for a long time, and I finally decided that now is a good time as any to start. Homemade 3D printed blasters have existed for some time, such as the Caliburn and FDL, but sourcing the hardware could be difficult since many of the parts are not offered in your typical hardware store. Thus, I wanted my own design to consist of only hardware that could easily be obtained. The design parameters I set were as follows:

• Spring powered for better performance ceiling and no need for batteries

• Bullpup design to maximize barrel length and minimize total length

• Comfortable stock to handle distance, which can be problematic with spring bullpup blasters

• Plunger volume comparable to a Caliburn

• Accept both full length and half length darts

• Favor hardware over 3D printed components for strength and build time

• Find all hardware in the local hardware store or Amazon

• Take common Longshot and hobby springs

• Performance similar to an equivalent Longshot

Initial Design

Like most designs, mine started out on paper. I made a few rough sketches of ideas I had, and fleshed out some design parameters and features from there. 

Initial sketch

From this sketch, some of the crucial features can be seen. The entire blaster is supported by two 3/4" angle irons, chosen not only for its rigidity, but because it is readily available anywhere. It also gives a smooth channel for any sliding parts to slide on, which simplifies construction. The 2" plunger tube and upper receiver sits on top of those angle irons. The magwell with a paddle release is behind the grip, and the foregrip is roughly the same distance from the main grip as the stock is. There is also an elevated rail to accept standard accessories. This also does show some of the different ideas I had in regards to plunger length, handle design, and overall aesthetics. 

Expanding on the mechanical design, the main mechanism is a bog standard floating plunger design. The plunger head freefloats inside the plunger tube, and the prime pushes the head back and compresses the spring to the point that it catches. The catch is a standard sliding plate catch, similar to most stock Nerf blasters. Once the plunger head is secured in place by the catch, the back stroke of the pump moves forward, which gives the plunger head space to move forward. When the trigger is pressed, it releases the catch, and the spring tension propels the piston forward, compressing air into the barrel.

For the barrel, I decided to initially go with a brass barrel, since it is compatible with both full length and half length darts, and it was a design I was familiar with and had plenty of. It is of a standard Sleeper breech construction, with a 9/16" brass breech telescoping into a 17/32" brass barrel. Barrel length is nominally 12" long because it's easy to find foot sections of brass, but it can easily be expanded past that. The barrel is shrouded by a section of 1/2" PVC, which not only holds the brass in place, but also protects it from any dings or impact.

A unique design feature of the floating plunger is the novel way it decelerates at the end of travel. There is a wide rubber washer roughly 1/3" in front of the actual plunger head that interfaces with the conical breech, which not only decelerates the plunger by deflecting, but also forms an air cushion in the last 1/4" or so to mitigate impact. The result is a lighter plunger, only requiring 3 walls and 20% infill, which can be dry fired with an 18kg spring without cracking.

After the sketch, I started working on a CAD model in Inventor. This is one of the early design iterations:

An early snapshot of the design

As you can see, a number of features have changed. The most noticeable change is that the entire grip is featureless and integrated with the magwell, rather than the original pistol grip. This change was from the model making process, when I found that once things got dimensioned, the grip would be too close to the magwell, which leads to an uncomfortable hold and pinch point when accessing the magazine. In addition, in other homemade blaster designs, the pistol grip was sometimes a weak point in the 3D printing process, causing breakages when priming the blaster. This new grip design mitigates that issue.

The actual design process took one or two months, and after some design changes to account for hardware choices and print efficiency, I got to the point to actually make this thing.

Prototyping

The first prototype is always one of the most challenging to build. It's this step where you're not sure if everything will actually work according to plan, and there's no prior knowledge on the build process in the order everything should go together. I had to get a few new tools, namely a drill press and tap set, to build it properly. The 3D printed parts were the easiest portion, and the hardware was definitely the trickiest to tackle. Cutting a channel for the magazine out of the angle iron was difficult at first, as I was trying to use the dremel with a cutoff wheel, to varying degrees of success. In later prototypes I refined the process to using a hacksaw and an angle grinder. After a long build period, the first prototype was complete. It's finally alive!

Comparison with a Nerf Kronos, with Worker SCAR attached to the front

Ok so that probably was pretty underwhelming. This was an airflow test to see if the plunger could move air, and used a 2kg Recon spring and no barrel. Very quickly, some issues became apparent. Most notably, air was leaking out of the front ram as the O-ring gland had not been sized properly, reducing efficiency and preventing darts from coming out of the barrel. The spring also kept buckling behind the plunger, blocking the hole for the plunger rod to go through. These are some issues that I had not considered before the first test.

The other problem was the front grip shape. I had originally modeled it after a semicircular pump shotgun grip, but it was very difficult to grab and pull back at the much higher spring loads for Nerf blasters. This ergonomic issue was very difficult to correct in CAD, as it basically required printing out the part, and getting a feel for how comfortable it was. It took me roughly eight iterations to get to a handle design that was comfortable yet strong.

A couple of grip designs I tried, with the final on the bottom right

Final

After many changes from the initial prototype, the final design was locked in.

Final CAD profile, with spare mag holder in the stock

Compared to the first one, several changes can be seen, many of them ergonomic. The rail sits slightly lower, giving it not only a more aggressive look, but also aligning a RDS closer to the eye level when the cheek rests on the back. The pump grip was changed to a square design, that spreads force evenly and is easy to grab onto. The screws were also changed from #10 to #8 for greater availability. The hole positions on the rail were slightly shifted to line up with ruler markings to make it much easier to mark out and drill. Lastly, the entire thing was given the weight loss treatment, allowing the entire thing to be printed with less than 400g of plastic. 

First prototype, in rear, and final prototype in front

Performance is definitely strong. Coming in under a total length of 24 inches, it features a barrel length of almost 16 inches, which truly makes it one of the most efficient bullpup spring Nerf blasters out there in terms of barrel length to total length. It's extremely maneuverable in tight spaces, and the featureless handle actually helps a bit in pointing the blaster. The entire blaster is SOLID, with absolutely no flexing or warping at all. It takes both regular Nerf darts as well as half length darts using a standard magwell adapter. I have it running an 18kg spring, and it fires well over 150 feet and even outranges my brassed Nerf Longshot with the same spring load. Unfortunately, I do not have a chronograph, so I don't have any FPS numbers, but it is definitely a force to be reckoned with, especially for its compact size. 

Conclusions

Overall, there were many lessons and takeaways from the entire experience building the blaster, but I have highlighted a few of the major ones. 

• Ergonomics can be tricky to design on a computer screen. It really takes an actual, tangible product to really get a feel for how well it will do.
• Tolerances are something to control for. Some places like screw holes can get away with slightly looser tolerances. Things like O-ring glands cannot.
• Using the right tools for the job can make a huge difference. I spent the same amount of time trying to dremel a section out of the aluminum angle iron and wasted a cutoff disc for a worse result than a simple handsaw cut. 

This was quite a fun build over six months, and I'm excited to share it with everyone as well. Detailed instructions for how to build your own can be found here, and the 3D printed parts files can be found on my Etsy so you can print your own. 

Image Gallery

Side profile

Rear shot

Rear side profile, showing the catch and trigger rod

Front offset profile, showing the prime bars

3D render of the completed blaster