Printable Airsoft Submachine Gun

After designing my first 3D printable airsoft gun, a few people have been requesting a smaller one that they can print at home. So here it is!
All parts are small enough to fit comfortably in a 120x120x120mm print volume, there are fewer non-printed parts, and it has more features. The gun won't be as powerful as the previous design due to it's smaller size, but on 0.12g bb's it'll shoot 25-30m.

STL's can be downloaded as a zip below.
You'll also notice an animation. If you're not so confident you can follow the instructions here, take a look at the animation. It is an exploded view disassembly of the gun, so you can pause and check you've made yours correctly.

Here is a list of the non-printable parts you'll need:
► 3 x M4x25mm* countersunk bolts
► 1 x M4x25mm buttonhead bolt
► 2 x M4 nut
► 1 x M4x38mm countersunk bolt
► 1 x rubber band between 50mm-100mm long (measured unstretched flat)
► 3 x 5-6mm OD* 20mm long pen springs (any pen spring will do)
► 1 x 5-6mm OD 60mm long pen spring
► 7 x 2mm OD 20mm (or longer) nails
► 1 x 2mm OD 60mm long flat head nail
► 2 x 1.5mm OD 20mm (or longer) nails
► 1 x Bike Pump*
► 1 x 190mm airsoft barrel & hop up bucking
► 1 x 1m of aluminium flat bar (12mm x 3mm cross section)
► 1 x 700g (or more) reel of ABS printer filament (black recommended)
► 1 x 100mm long bb pistol spring, or any spring of your choice (15mm OD)

And here are the tools you need:
► 3D Printer with 120mm cubed volume
► Pliers/side cutters (for cutting nails)
► Support removal tools
► Sand paper
► Phillips head screwdriver

*M4x25 means 4mm diameter (outside of thread) and 25mm long
*OD stands for 'outer diameter'
*Here's a couple of links to a similar bike pump:
https://www.ebay.com/itm/Mini-Bicycle-Pump-Portabl...
https://www.aliexpress.com/item/32973595341.html?s...
https://www.ebay.com/itm/Mini-Bicycle-Pump-Portabl...

Step 1: Features

Before you start building, it's good to have an idea of what the final product can do.

Here are some features:

• Foldable and removable stock
• Working safety
• Twist to remove barrel for quick unjamming
• Easy access adjustable hop up
• Folding foregrip
• 13 round mag
• Quick change spring
• Removable backplate- piston access
• Single-pin field strip
• Anticlockwise thread on over-barrel silencer
• Weaver rails, will fit standard attachments

For all the engineers out there, here's a brief overview of the design:

The focus of this design project was on assembly. The entire gun can be broken down into it's basic components with the removal of only 2 pins. This is possible because of sequentially constrained parts, where a pin holds one part in place, which in turn holds another part in place, and so on.
It's an original design, I used Solidworks to CAD it up, and an UP Plus 2 to print it. One of the challenges was to design all the parts to fit within a 120mm cube print volume. This required major parts like the receiver to be split into 3 parts, creating the need for mechanical connections between these parts. So I had a lot of fun finding creative ways of assembling these components without increasing the use of non-printed fixtures.
The design process (concepts through to CAD) was spread over about 6 months due to university and work, but overall took about 3 weeks of full time work to complete.

Step 2: Printing and Collecting Parts

Get a hold of all those parts I mentioned on the first page while you're printing. STL's can be downloaded from the first page if you missed that.

You'll notice I have a reflex sight there as well, this is optional as the gun is printed with sights.

I harvested my springs from dead BIC pens, but any pen springs will do as they are all similar sizes. Cut them to suitable sizes when you come to assembling the parts.

The piston spring came from an old bb gun, but you can find similar springs at a local hardware store if you don't have a spring lying around.

Cut the cylinder of the bike pump to 50mm long and deburr the edges. Take the o-ring off the piston, and try and keep the grease so you don't need to re-lube your piston.

Step 3: Assembly- Folding Foregrip

Pictures are pretty self explanatory for the next few sections, but let me know if you have any questions. One thing I will say though, keep the sand paper handy, you'll need it to get these parts moving smoothly.

Step 4: Assembly- Hop Up

This hop up unit is a little tricky to assemble, but again, the pictures do a better job than me explaining it.

Step 5: Assembly- Trigger Box

The trigger box is split into two parts, one holds the sear, the other holds the trigger. A link is inserted into the receiver later on that will connect the two.

You'll notice there are no photo instructions for the trigger side of the trigger box, but it is pretty simple to figure out. Check the animation if you're stuck, and if you still can't get it, just leave a comment and I'll see if I can help.

Step 6: Assembly- Mag

In the previous gun, a few people had trouble finding an old bb gun mag for the spring. So instead of a spring this time, I designed the mag to use a rubber band. Use some common sense here to find the right length of rubber band to cut, the ram should still have tension on it at the top (when there are no bb's in the mag). You will need to use some sand paper to get it sliding smoothly.
To thread the rubber band through the body of the mag, I use a piece of wire.
The nail used for the pulley wheel here has a 1.5mm diameter.

Step 7: Assembly- Stock

Making the stock is optional, but I think it looks cool.

The aluminium flat bar will need to be bent into shape. You can make it any length you want, mine is 260mm from end to end. So first cut a length of bar to (2xLength)+44mm, where Length is roughly the distance of the stock from end to end, and 44mm is the internal width. Mark off the centre and measure 22mm either side as your fold lines. Place the bar in a vice or grab it with the pliers at your bend lines and bend a touch over 90deg so the ends point inward when you're done. This will help them clamp onto the receiver.

Step 8: Assembly- Barrel

If your barrel is a little shorter than 190mm that's fine, it will still fit, but over 190mm will stick out the end of the outer barrel!

My hop up bucking was a cheap fake madbull, but most are standard sizes so you can get any brand that you want.

You can print a flash hider as well if you don't like the silencer, both have anitclockwise thread and you can change them out easily.

Step 9: Assembly- Receiver

Pics should show everything, except the assembly of the bolt and rails.
Once you're cut the cylinder to 50mm and deburred the edges, use some epoxy to glue the cylinder head into one end. Cut a 1.5mm diameter nail and pin the fin into the cylinder head as per the picture. The 60mm long flathead nail should clip straight into the top of the clinder head.
The rails on the receiver are simply acetone fused in place. However, if you have some spare epoxy, you may wish to do this at the same time.

Step 10: Assembly- Putting It All Together

Now for the fun part, putting all these sub-assemblies together! Follow the pics, should be pretty easy, but do let me know if there are any details that I have missed.

Step 11: Painting

I used a slow cooker with some acetone to smooth the surface of the parts. Make sure they fit together well before you do this however, or you'll need to sand them again and defeat the purpose of the acetone.

I spray painted my gun black (with white primer), but the paint had a slightly sticky feel to it, causing some parts to grip tightly. I would recommend printing in the colour you want your gun to be so you avoid painting. However, I was able to sand the edges to give a worn look. Again, up to you!

Hope you enjoy this building this gun if you choose to, make sure you use it responsibly as always. Take a look at the parts understand how they work together, I hope it will inspire you to get into some CAD and start designing!