Step 1: Acquire Parts
I personally only had two tanks (due to lack of funding) that were rather large. The gun is a little awkward to move around, but if you are waiting for your victim to blunder across your line of fire it is lethal.
The parts i used:
3/4' electronic sprinkler valve
1/2' pipe (for barrel)
1' pipe (for manifold)
3' pipe (for tanks)
2 metal ball valves
3 1' T-connectors
3 1' male adaptors
1' slip cap
1' female threaded cap (so that if i wanted to i could add another tank)
1'-3/4' male adaptor
3/4' male adaptor
1/2 female adaptor
1/2 male adaptor
2 1/2 couplings
2 garden hose rubber washer hose thingys
1 threaded car valve (so you can pump it with a bike pump or air compressor)
Step 2: Construct Manifold
Next, connect all your 1' t's to your 1' pipe. Make sure you leave enough space between them to fit the tanks.
On top of these t's glue on your 1' male adaptors
on the front of the manifold glue your 1' to 3/4 threaded adaptor
At the end glue on your slip cap with the pressure valve on it.
Step 3: Construct Tanks
screw or glue on your your ball valves (mine are nice ones, i had them left over from a previous project. This kind of investment is not necessary and the pvc ones will work fine)
it helps if your ball valves are threaded so you will not have to glue on a female adaptor before you attatch it to your mainifold.
Step 4: Construct Barrel
Glue a 1/2 male adaptor to your barrel. This will screw into the assembly we just created.
Now duck tape together your two 1/2 couplings. Then drill them out so the fit easily over your barrel.
Using a dremel or drill create a gap large enough at the bottom of the barrel so you can load the darts directly into the bottom without using a ramrod. I put two hose rubber thingers on either side so no extra air can get out.
You use the coupling to slide over the gap you created while shooting.
Step 5: Misc
If you would like to use an electric switch for your gun you could make a simple circuit using a few nine volt batteries and a light switch. Mine tended to lag but i only used one nine volt (which was already half dead) and the system supposedly required 24 volts.
You will find that standard darts are not heavy enough and that you would be better off making your own.
I used 1/2 foam backer rod ( i picked it up at Home Depot, ACE or True Value probably carries it as well) as the primary body of my dart. I found that it did not fit in the barrel as tightly as I hoped. I am going to try using 3/4. For wieght I screwed in a small 1/4 inch screw in the front and put a little pink duct tape around it so they would be easier to find and retrieve.
Expirement around until you find a dart that works for your gun. You can eliminate the slight arch in the foam backer rod by putting them in a pillow case and turing a hair dryer on in it for a few minutes.
I believe that if you could figure a way to sturdy up the barrel of this gun (the 1/2 wobbles around) that you could greatly increase the accuracy and maybe the range.
The main problem I am having is stability. I need to build a stand of sorts (maybe a bipod?) that will allow me to more easily adjust the firing angle. Any Ideas? Thanks