BAX Adjustable Hop Up.
I got adventous and dismantled my DPMS Evolution far enough to get a gander at the internals of the BAX system.
There are two or more gears that turn a cam in tiny increments. This cam limits the downward movement of a springy lever/arm that apparent presses down on a rubber sheath around the chamber.
Since no .20 or larger BBs are available locally I've been using .12 BBs.
Its nice to be able to set the hop up to handle various weights.
I plan to get some .20 BBs one day this week.