For suprisingly little money and effort, you can convert a single shot airsoft gun into a rapid fire BB hose with an insane rate of fire.
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
The airsoft gun you choose for this build should have a 'T' style hopup unit. This type of hopup unit is internally shaped like intersecting tubing in the shape of a 'T' . Many spring shotguns and some low-end electric guns have them
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.
If you have access to an air compressor, that is ideal, but for those of you without one, an air supply of sufficient power can be made or aquired relatively inexpensivly. Our objective here is to have a large supply of air behind a 'momentary' valve, (meaning a valve that will only release air when it is held down). The air source can be attached to the valve in any way that is convinient. I would suggest that you go to your local hard ware store, and look at what you have available to you. You could even dry fit the various parts in the store to find out what would fit.
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
Now you need to find a way to put the air from the momentary valve through your hopup. I used .25" flexible tubing glued in the back where the air nozzle used to be, but you could similarly use something like a pen tube. Be sure not to obtsruct the opening through which BBs come from the magazine. The other end of whatever tube like ogject you used needs to be attached to the output end of your valve in a reasonably airtight manner.
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.