A cloud chamber BB machine gun, or BBMG, works by blowing air up through a mass of BBs (BBs are small plastic or metal balls). This makes a "cloud" of BBs inside the chamber in which the BBs move around at random, and occasionally get near enough to the entrance to the barrel to be sucked in.
The actual cloud chamber is typically built within a short length of small diameter pipe, such as a couple inch segment of 1.25" sch 40 pipe. More detailed descriptions can be found here However, cloud chamber launchers have been built to accept ammo up to 3/4" projectiles.
A cloud chamber BBMG typically gets rates of fire of about 50 projectiles per second, which is at least as good as the vortex block style of construction.