Algodoo

A gun that uses a build of pressure, via spawned in circles, to provide force to a projectile.

The gun operates by, more or less, heating up a gas to a critical temperature, before unlocking the bullet, which is serving as a tap otherwise.
The hot gas vents down the barrel, pushing the bullet.

Operation instructions are in-scene.
There's 3 towers rightward to shoot.

Update:
Made reloading more slick, and easier.

-Warning-
This does cause a substantial drop in fps whilst the gas is heated in the chamber.