That looks like a spud gun.
Once built, it will go in the chamber of a 7mm Rem. Mag.
The inside of the purple chamber will be pressurized to 3600 psi.
The firing pin will strike the valve, releasing the air, which will travel around the outside of the purple chamber and push the pellet out the barrel.
It should get around 1000 fps with a 25 grain pellet.
(According to my program that simulates internal ballistics for an air rifle.)
I think I will probably need to redesign the valve so that it is self actuating.
(I’m not sure a regular firing pin can open an unassisted valve with 3600 psi on it)