That would indicate considerable inexperience.
Actually, I worked with pneumatics for a couple years via FIRST robotics when I was in high school
You have failed to mention the second solenoid needed to charge the firing chamber of calculated size and pressure prior to firing.
Why would I have a secondary firing chamber? I can literally have direct release from the tank I'm using to store the air. A flow regulator will allow me to fine tune the output PSI. To help you envision the system (since I have no clue what you think I'm talking about) it'll involve a pressurized air tank feeding through a flow regulator to a solenoid, and out through a union tube to a number of PVC tubes capped at one end.
just pressurised air could mean anything.
Well, in this case, it means air that is stored under pressure.
Get it wrong and apart from the danger, under UK law you could end up being charged with 20 separate counts of possessing an illegal firearm
Well, I'll keep that in mind if I ever decide to take this to the UK. About the only real danger this device might realistically have is if a dart hit someone in the eye but that's true of any Nerf gun. It's not like these are being shot at 340 m/s. The flow regulator will allow me to control the pressure output of the system.
You can legally posses a crossbow or bow (massive spring) capable of taking down a bear , if that's your thing.
I can comfortably say this will not be taking down any bears.
Someone close could sustain serious injury both from the dart and air blast.
I would be quite surprised if that were actually the case. In addition to the flow regulator, you also have to factor in that the air is being released out of many barrels at once. It's going to be pretty widely distributed.
So, I do in fact appreciate your safety concerns. Safety is always paramount when building any project. That said, between the low mass of the darts, the wide distribution of pressure through multiple tubes, and the control of the output pressure, I think your fears are largely unfounded for this project. I'm reasonably confident I will not be being brought up on possession of lethal weapon charges any time soon. I'm more concerned with how to safely drive the solenoid without frying anything by accident and making sure that the solenoid I buy is capable of being driven by the Arduino.