The concept is simple: put a constant supply of high preassure air through the gun, and bbs will simply follow the airflow out the barrel. This means that this gun will fire as fast as the magazine can feed.
I recommend you get the highest capacity magazine you can find for your gun. Mine goes through its 30 rnd mag in about half a second (no exageration).
Step 1: Selecting a Gun
Step 2: Open It Up, and Gut It.
Its up to you how to figure out how to open your gun, as this could be done with any number of guns, which open in as many different ways.
After you have it open, take out any part that is not related to the hopup, barrel, or magazine This means you should leave in the magazine catch (the part that holds the magazine in), anything that helps to hold the barrel or hopup in place, and any levers or slides used to adjust the hopup.
Step 3: Aquire or Create an Air Source.
An easy valve to use would be an air compressor specific 'blowgun', but any momentary valve will do. I used an preassure washer handle and enclosed valve, because it had a nice trigger system ready to go.
For an improvise airsource, you could use 2 inch pvc pipe with a cap at one end, and reducer fitting on the other, which would attach to a blowgun. (explained below)
You could also use a bike tire co2 quick inflator, but you would spend a fortune on 12g co2 cartriges.
Step 4: Airsouce to Hopup
Step 5: Load and Shoot.
Now just mount your valve and gun together in whatever manner is convinient, charge up your airsource, load the gun, and open the valve. If you did it right, you should hear a fart like noise as you send your entire magazine worth of BBs down range faster that you thought possible.