Credits for the label go to aonomus. I Just rearranged the contents to fit the 89x42mm labels which I use (Software: Dymo LabelWriter to open the .label file).
Label source: https://www.thingiverse.com/thing:2757691

Attention:

This is a "working" auto-injector. This is NOT A TOY!
I am not responsible for ANY injuries caused by building and operating this design!

Update 2019-11-03

• added openSCAD V2 sources
• added cardboard packaging design (print in 600dpi, use this file: https://thingiverse-production-new.s3.amazonaws.com/assets/e2/ce/d6/81/43/Zombrex_Packaging_V2.png)
• refined some parts (see V2 files)
• "Needle Guide" isnt required any more, instead you can use "Needle Bumper" - my centrifuge tube broke after 8-10 uses since the syringe needle plastic got rammed into the front hole. The bumper fixes that issue while simultaneously acting as the needle guide
• added little demonstration video: https://youtu.be/K09eIKxpLcM

NOTE: Zombrex needs to be injected to the body instead the bottle to work properly!

Update 2018-11-26

added photo or the "real-world" assembly

BOM:

1x 3ml syringe (concentric needle)
1x polystyrol centrifuge tube 100 x 16mm ("Böttger Zentrifugenröhrchen")
1x needle of your choice, i prefer 1mm needles
1x steel spring OD 11mm, length 60mm, compressed length max 20mm

Preparations:

Cut the syringe housing at the 2.5 ml mark - syringe housing should now be 45mm in length (from the tip to the cut).

Cut the syringe plunger to 40mm length.

Assembly:

First assemble the syringe with the needle guide.
Then load the spring between the syringe base and endcap - plug in the safety cap to not fire the thing by accident ;)
Then plug the syringe shaft into the spring assembly.
Now load 1ml fluid to the syringe and finally plug the whole assembly into the tube. - > DONE and ready to "unload" into something....

After Assembly:

Cut the needle to your preferred length (should definitive be inside the tube on loaded spring).
Assembly is designed to hold 1ml of fluid in syringe.

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