SpudFiles

In my opinion, you can't go wrong with a big chamber if you have a decent sized air compressor. If all you have is a bike pump, I'd go along with the suggested length from the GGDT. To check for projectile range, click "Tools" (up the top), then "External Ballistics Calculator". If you tick the max range box, it will work out the best angle for you to shoot at if you want a long distance. (Glad someone finally answered your question?)

my main worry was about the barrel once the air goes throught a smaller hole does it lose power. what i mean is should i have the barrel the same size as the sprinkler valve?

My above post was in reference to frogy's questions. Regarding your question (Tyro) A smaller barrel will restrict airflow, but using a larger diameter barrel will allow the air to loose pressure much faster because of the increased volume. If you have a very long barrel with a small diameter, then you will achieve the highest muzzle velocity (I think, I'm just using logic to explain that, not numbers)

so same size pipe as the valve would be good, Would 1 metre be enough?