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Discussion Starter · #1 · (Edited)
I've had this idea recently. The air chamber has pretty much zero volume when unloaded. As it's filled with air, the chamber is expanding while pushing against a counter force, like a coil or gas spring. (I've had more ideas in mind, including a mechanically squeezed high-pressure hose for example)

Think of a split pneumatic cylinder with two chambers and a short piston in between. The left (rear) side contains a counter pressure that's below the working pressure. When unloaded, the piston sits all the way on the right end, closing up the right part (air chamber) pretty much entirely, with only a small gap to allow air to get in. As the right part is filled, the air will make itself some room by pressing the piston backwards. I thought that, since the chamber "shrinks" while the air is released, it would probably retain a higher pressure or maybe reduce the required air volume. What do you guys think about this?
 

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Discussion Starter · #2 ·
On further consideration, I think the counter pressure side should have a fairly large volume, so that the piston movement has little influence on the pressure of the counter pressure side.
 

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Discussion Starter · #3 · (Edited)
I also think you should use a rather large diameter for the air chamber, to reduce the travel and speed of the piston, hence enabling it to "follow" the pressure drop faster. A large, but flat air chamber.
 

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Discussion Starter · #4 · (Edited)
The piston also has to be build as light as possible of course. More like a disc. And the gas on the counter pressure side should probably be a lighter one, like helium. Not sure if this would have an influence though...
 

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Discussion Starter · #5 · (Edited)
This is all just theory, guys. I also have a really hard time to wrap my mind around this whole thing, to be honest, haha. Let's take the idea apart. The "empty" state is kinda "pre-loaded". Let's say the counter pressure is set to 500 psi. This should mean that the pressure within the air chamber and barrel never can drop below 500 psi, until the air chamber is completely "gone" and only the volume of the barrel itself is left. Provided that the piston is able to move fast enough to keep up with the pressure loss, of course. I'm not entirely sure about the influence on the required pressure or volume, but I think you should be able to achieve at least a more consistent expansion through this. The pressure drop behind the bullet should be decreased.

In other words, the idea is to compensate for the volume increase of the barrel (through the bullet traveling forward) by decreasing the volume of the air chamber simultaneously. (through the counter pressure) I also could imagine that you have to find a specific pressure ratio to maximize the benefit, or even achieve any.
 
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