Break action rocket launcher that shoots rockets propelled by co2 cartridges. Rockets are held in reusable shells and upon firing, they deploy stabilization fins thanks to rubber bands.

The rockets use 12g co2 cartridges which are often used for airsoft. That makes them quite affordable and available in any airsoft store.
Launcher features picatinny rails on top and bottom of the barrel, making it possible to mount accessories like scope or laser.

For more information and assembly guide see assembly.pdf in files.

BOM:

• M3x8 7pcs
• M3x60 2pcs
• M3x35 2pcs
• M3x25 1pc
• M3x22 1pc
• M4x25 1pc
• M4x22 1pc
• M4x10 4pcs
• M4 nut
• Springs 5,5mm OD, length at least 30mm (Springs from HORNBACH)
• Torsion spring - wire diameter: ~2.5mm, inner hole diameter: 8mm, outer diameter: 13.5mm, height: ~13mm (I got mine from this clamp)
• Nail 2.8mm x 40 (HORNBACH link)
• 12gram non threaded co2 cartridges
• rubber bands
   

For additional types of rockets or if you want to create your own see
Co2 rockets (Makerworld link)

In the files section you can find honing tool. This is not needed but can be used to smoothen the inside of the barrel. Simply glue sandpaper to it and insert 8mm tube into the center. Mount it onto a drill and use it to smoothen the barrel.

UPDATE: Added shorter 167mm barrel so it can be printed on prusa mini or A1 mini.

Thanks to @Ice_Maniac, who found the correct set of springs and clamps on amazon:
Clamps from amazon
Springs from amazon