We have a radish cannon that is about a foot and a half long, and shoots baby carrots real nice. We also have a few 6 foot long 1.5" diameter tube 3" chamber with grill sparker cannons.
I recomend skipping potatos down a stream or across a lake.
A warning. The PVC will get very brittle when it is cold. If you keep it un heated, take it inside and warm it up before firing it at 30F. We shattered one firing it cold. I was surprised noone got hurt by shrapnel.
Wetzel