SpudFiles

The 4" round air chamber totally surrounds the 2" barrel. The Barrel goes all the way through the air chamber. The reducer at the both ends of the gun had to be carefully sanded so the 2" could pass all the way through the reducer and still be air tight.
This is a lot less awkward to carry.

Wasn't it easier to make a piston valve cannon? .-.

Creative design...no benefit, huge dead space, restricted flow...but creative.

If I said "DWV"...would that mean anything to you?

What is your intended working pressure?

I've actually seen this configuration before but kudos if you came up with it independently

As Gippeto pointed out, it would have been much more powerful with the sprinkler valve fitted at the reducer where you schrader valve is, but hey, if it works it works, nice job!

What JSR said.

Just do exercise extra care handling that thing.

Seen it too, but it's still cool.

Would be cooler if you used pressure rated parts and painted it

You could have placed the valve right behind the coaxial chamber.
Or put a breech inbetween.

That would have taken out a lot of dead space.

Kudos for trying something different right away though!
Most start out by copying others or by asking for designs.

You could insert a smaller diameter barrel and use the dead space ( the elbows loading up to the straight barrel) to hold some additional ammo.

plastic bb's would work without tearing up the inside...