I decided to use a 1.5 inch porting piston valve instead of just a sprinkler valve or 2, mostly because I was given unlimited funds. I used a piston made of 2 1.25" end caps, with o-ring grooves carved with a dremel. The sealing face was a simple neoprene washer, and I used a cone shaped block of wood to center the piston on the sealing pipe. I can draw a diagram if that is confusing, but I think I saw someone use it here.
I wanted to keep the 2' long by 4" diameter chamber small so it wouldn't look weird and it would use less CO2. It's just a simple chamber.
I also made a breech loading hole with a modified coupler to cover it up. I carved with the Dremel (carefully) an L-shaped slot on the outside of the barrel for a screw to slide in, so it cocked like a bolt action rifle (sooo cool). We would grab 4 handfull's of confetti and scoop of flour (for a smoke effect), close the breech, and fire. It was so cool, I wish I got a picture of it firing. If anyone has ever shot confetti, you know what I mean. The barrel was the full 10 foot piece of 3 inch PVC with 4 inch PVC sleeving it, so it would look bigger (it ended up to still be too small, so we wrapped it in foam, looked awesome).
My CO2 setup consisted of a beer keg/soda dispenser CO2 regulator, and I used 20 ounce paintball CO2 tanks. I hooked up an ASA (paintball tank to 1/8" NPT adapter basically) to the input of the regulator, and screwed in a new tank when the old one was empty. I did this because I knew it would be too cold for the CO2, so we kept the unused tanks in the car to warm up. When we needed a full one, we just asked the driver to pass back a fresh tank (we had 5). I tried using a much simpler paintball regulator, but it didn't have enough flow to seal my not-so-perfect piston valve. This beer keg reg worked incredibly fast and after it was set up, it worked great. I suggest this to anyone who doesn't need a small or good looking CO2 set up. They are sold on Ebay for less than $50. I had 10 feet of tubing and a ball valve to control the filling.
Well, it sounds like a solid setup, not too fancy, right? Well, it was so cold (or the CO2 froze the PVC
) that the piston cracked, the sealing face was shot out about 6 inches into the barrel, and it rendered it useless. A simple thing that I never even thought of thinking of. Of course, it broke one block before the judges stand. I think I'll use nitrogen next year 
.EDIT: I thought I had more pictures, I'm sorry.
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