Advice for pneumatic speargun

[mdelica]

Hello everybody!

I am looking for advice regarding a project I would like to build: A pneumatic speargun.

There are already some of those on the market, but they have a lot of shortcomings (maintenance, vendor parts, custom inflation valves, poor quality etc).
The inner workings of these is similar across brands, and can be understood from this video:

I came up with two different ways of building an alternative, one resembling what is usually being built with QEVs.
The gun looks like what is in this picture: https://postimg.cc/WtQQgcxt
The gun is called a Pelletier Co2 gun and is illegal in most regions now because it is not recharged by human force.
I'd like to build something similar, but rechargeable.
After reading a bit on the topic, I still have some questions and am looking for general advice on how to best build this.

Some specs:
• Operating pressure is usually around 20-30 bars (for it to be legal I have to be able to charge it manually)
• A piston enclosed within a barrel (Al or Stainless Steel) will push a 7mm shaft over its length
• Device will be recharged by pushing the piston back and should not lose air like a conventional pneumatic canon

Some desires:
• Something more powerful than what is on the market (pressure has to be released as quickly as possible).
• I like the idea of having a barrel that's not enclosed in a pressure chamber. Idea would be to make maintenance easier.
• Normal pneumatic spearguns are impossible to discarge without firing, I'd like to be able to release air back in the barrel slowly.
• I would absolutely love being able to have an electonic trigger (normal triggers are hard, and require quite some debatement to release the shaft). Can sort of give up on that one, but maybe it allows for slow discharge.

Questions:
1. I am not sure I fully understand the difference between a solenoid valve and a QEV (I do get that all solenoids are not QEVs). Am I right to understand that QEV = type of valve and solenoid = actuator (the same way some QEVs are actuated by pressure) ?
2. Which kind of valve(s) should I favour for such a project, especially considering that the device has to be recharged by pushing back the piston?
3. What are the most common physics formulas that would help me calculate what the speed or energy of my piston will be?
4. Why do people mod QEVs with a spring?
5. What if I want that thing to be electronic, should I simply use a solenoid QEV?

I am aware water ingress is something altogether. Consider water not being the problem but rather the fact that the whole system is a 'closed loop'.

[jackssmirkingrevenge]

Welcome to the forum!

Interesting project, I suppose the first question before dispensing advice would be what is your budget and what are your technical abilities and available facilities?

Does it have to be made from off-the-shelf components or are you able to envisage some machined parts?

I'm not sure valves are relevant to such a project if you are compressing it manually, shouldn't you be looking at ways to retain and release the piston?