Hey, this is my second good instructable so I am still kind of new to the whole instructable thing. Before we get started, I want to promote and give thanks to the video that taught me how to do it. I acctually forgot to take pictures in the making, but its pretty simple so you wont need that much instruction. I will give little tips and solutions that the guy didn't say, so this isn't just a blunt copy.
Here's the link:
http://www.youtube.com/watch?v=FdV3nZOFgS0&eurl=http%3A%2F%2Fwww.wonderhowto.com%2Fhow-to%2Fvideo%2Fhow-to-make-a-high-powered-nerf-rifle-189571%2F&feature=player_embedded

Enjoy!

Step 1: What You Need

It doesn't take or cost much, but here is the crap you need.
2 feet 1/2 inch pvc (must be 1/2 inch to fire well!!!!)
1/2 inch plastic pvc ball valve
ball valve
basket ball pump/tube
dowel
2L bottle
gorilla glue
duct tape
file
pvc cemet

can be 5 to 20 dollars, depending on the quality, and amount

Step 2: Pvc Stuff

Cut a piece of pvc about 5 cm. File down one end so its a lot thinner. Connect that to the 2L bottle (make sure its empty). Connect the non-filed end into one end of the ball valve. Put 6 to 18 inches of pvc to the other end of the ball valve. Use pvc cement to secure it.
I used 1 inch pvc much to my despair, and than rolled up paper to make it thinner, i do not recommend this.

Step 3: Put in the Pump

Put the pump hose in the bottle so it barely fits (use a drill), gorilla glue it a hell of a lot to get an airtight seal. Here is a list of things not to use for the seal:
Clay
Elmers Glue
Caulk
Duct tape
Nothing
Your finger