haha. Well I cant provide any evidence that these range claims are precise. However, Ive seen this cannon fire, and I can definitely say that I would believe it. And just in case you are curious. <A href="http://maps.google.com/maps?f=q&hl=en&q=holtville+ca&layer=&ie=UTF8&z=18&ll=32.884394,-115.316062&spn=0.003167,0.006781&t=k&om=1">This</a> is a Google picture of the farm/field that the cannon is being shot from (assuming he "hi" hasn't relocated the cannon to the house to the east). The field to the west is our usual firing range, and I would say that the potato easily cleared the first field and landed somewhere in the second field. The first field is about a 1/4 mile on an end, and the second is over a 1/2 mile. Therefore, it is quite believable that the potato went 1/2-3/4 of a mile.
Don't let his ball valve fool ya, this cannon packs one hell of a punch.