Ya the barrel should be longer, do you think that I would get a lot more range out of a longer barrel? And as for the valve the threads weren't long enough to go through the cap and pvc. I think it should be ok because it has a shoulder on the inside.

This is the potato cannon I made:
4"x18" chamber
4" Cap
4"x2" reducer with 2"x1.5" bushing
1.5" ball valve
1.5" Barrel
Screw in schrader valve
http://i52.photobucket.com/albums/g26/hascet/CIMG0018-1.jpg
http://i52.photobucket.com/albums/g26/hascet/CIMG0019.jpg
http://i52.photobucket.com/albums ...