Here is the first gun and all the discussion about it. As you can see, it was to big and it leaked.
So I decided to try my best to solve those problems on the next gun I made and thankfully I did. Here is my first succesful nerf cannon:

Range with nerf dart: roughly 400ft
Range with potatoe: roughly 250-300ft
It takes about a three minutes to pump up with my little hand pump and maybe one minute to pump up with a compressor. I am only running it at 30 psi right now.
Enjoy!
-sponge