[vivek16]

The ideal angle is 45 degrees. Half of your energy goes to propelling it forward and half goes to keeping it in the air. You will get the maximum distance for this.

Well, having a larger valve would increase air flow. Try to find a valve that is close to the size of your barrel. It would be nice if your air chamber were as big as your valve but not totally necessary. Make sure that your air chamber has atleast 1.5 times as much space as your barrel can hold (I think that would help).

hope it helps, Vivek

EDIT: fimmel is right, if the barrel is longer, the balloon has a longer period of acceleration. The air chamber larger than barrel rule still helps though. Water lubrication will be fine as long as you just keep the inside slightly wet.