Looks somewhat like a cross between the Pancor Jackhammer shotgun and the TDI Vector submachinegun, both of which are firearms of an interesting appearance, so no complaints on that front.
However, as for what pressure is needed for a given velocity, it does have a dependence on barrel length and valve, and to a lesser extent, specific chamber size.
We can say "ooh pretty appearance", but our primary expertise here is the functionality of these things, and to give feedback on that we do need more numbers and preferably reasonably detailed diagrams.
It's hard to work just from a paragraph of description.
Also, has a high capacity clip ever been made before? As in, one which you wind up?
Before someone less forgiving slates you for it, there is a difference between clips and magazines, and the terms are not interchangeable.
Clips are bits of springy metal that hold a round by the rim on it's base for feeding into a magazine. Magazines are those systems which take a direct part in feeding a round* into the breech, usually by a spring mechanism - they may be either integral to the gun, or removable and interchangeable.
9 (or more) times out of ten, when someone says clip with reference to a firearm, they really mean magazine.
*Arguably, round is similar term. Round only technically refers to the combined bullet, case, powder charge and primer together - it's common to see it used to refer to the projectile.