This comes to mind to shoot small steel balls, without using air pump valves or rupture disc. It is a short weapon format.
It is obvious that the mixing compression screw can be replaced with some compressed air. I will draw this option too. What seems "delicious" to me is not to use the damn air pump, and to have the cartridges loaded without pressure ... The prototype, (which I would like to try), would be compact and quite simple.
Sorry to Wyrlok for messing up your post, but if you prefer I can open another thread with these things. Likewise, perhaps, it will serve you the delirium we made with Jack here to consider some other alternatives to his idea.

Wyrlok wrote:Ok so we now have several options:

normal pressure butane or propane : not so much power so it's a no from me
normal pressure air without explosion : needs more pressure for the same power, not viable

Hectmarr's pressurized air/butane or propane, compressed when inserting the projectile in the shell/in the barrel
- retention mechanism may be a little difficult to make
+ is relatively easy to reload
Jackssmirkingrevenge's pressurized air/butane or propane, with valve to compress air (and gas that is already inside)
- shells may be a little difficult to make
- seems long to reload
+ seems somehow more reliable than the previous solution
i'll add normal-pressure hydrogen to the list
- need to produce hydrogen to fill the shells
- i could die
+ simple mechanism makes it easy to reload and reliable (except if the tube explodes)

Thank you for you suggestions, I'll try to see how I could make them to see which one is better/possible

Good analysis of what is provided here. I am inclined to Jack's design, because it is more reliable in terms of pressure retention inside the cartridge. That should work fine.
I suggest you try butane, (or propane), and air. Hydrogen is an extra complication in preliminary design phase and a distraction in handling, it can give a good scare ...
With a 3X hybrid and a 30 -40 mm ammo, you should have an interesting explosion and not too dangerous. Continue developing your project and show your progress when you decide.

Wyrlok wrote:Very nice design, and the epoxy is a good idea to insulate the shell. What would you use as a burst disk ? I guess it has to be strong enough to contain pressure but weak to break when it explodes.

One of my favorite burst disk materials was photo paper, it held pressure well and broke nicely when subjected to the full pressure of the ignited mix. There are other materials you can use though, aluminum foil is another favorite. You add or remove layers depending on the pressures you are using.

What a coincidence I have a set of 10 schrader valves coming from china in a week or 2. I didn't need them but I knew it could be useful some day. Wouldn't epoxy "unglue" from the shell in case of explosion ?

Epoxy construction can be very tough if done properly with the right materials.

For the sake of completeness, here are examples from yours truly of when it goes well, and when it doesn't. Learn from my mistakes

i'll add normal-pressure hydrogen to the list
- need to produce hydrogen to fill the shells
- i could die
+ simple mechanism makes it easy to reload and reliable (except if the tube explodes)

One way to boost power without resorting to higher pressure is to use higher concentrations of oxygen. In a normal atmospheric pressure hybrid, what you have available is around 20% concentration of oxygen, so you can only use around 3% propane. If however you use pure oxygen, then you can effectively make a 5x hybrid without the need to pressurize.

The reason I didn't mention it previously however is that pure oxygen is quite a dangerous thing to be handling.

I suppose if you're using oxyhydrogen that you've generated from water through electrolysis at atmospheric pressure in small quantities then it is a safer option, but not one I have explored personally.

hectmarr wrote:This comes to mind to shoot small steel balls, without using air pump valves or rupture disc. It is a short weapon format.

I considered something similar here with the pressure itself holding the o-ring but never tried it:

I considered something similar here with the pressure itself holding the o-ring but never tried it:

Image

Yes, I have seen the design carefully, however I am inclined to use a simple rupture disc in the cartridge. The problem of this type of retention is complicated to regulate, I think you agree with this, apart from that the o'ring seals slowly degrade in each shot. I have seen this issue in this type of retention that I have installed in 3 of my autonomous hybrid.
If I build a cartridge hybrid, a repeat weapon, I would do it as you said before. A long job because basically I have to build 5 equal small hybrids, at least, which are the cartridges.
I have modified your drawing based on the needs of my applications. I have no doubt that it works.

hectmarr wrote:If I build a cartridge hybrid, a repeat weapon, I would do it as you said before. A long job because basically I have to build 5 equal small hybrids, at least, which are the cartridges.

That's basically why I never went through with such a project, always trying to find ways of making it simpler since it needs to be made in quantity. Sometimes though it's easier to just suck it up and go through with the effort into something perhaps a little more complex but that works well.

It would be a master weapon ... it can even be done semi-automatically or repetitively, in its humblest version. It is a project for someone with MANY desire to work! jajajajajaja