Oh, dear. Do you have a lot of land? Do you have friends who have a lot of land? (A lot of land, like a vacant area at least a half-mile on each side.) Also, do you have really great insurance?
Given the news of this year's relative scarcity of pumpkins, maybe "punkin'-chunkin'" isn't a great idea anyway. But if you are not to be dissuaded, and you have the land, insurance, and expertise, you will want to see this giant pneumatic cannon over at Instructables. Not only do you get all the information you need to build one of your own, you also get demo videos to show off the kind of damage it can perpetrate.
Whenever you build something, safety should be your number-one priority. It seems pertinent to stress that point in this case. It's not actually under the "not liable" keyword at Instructables, but it's probably a candidate for inclusion.
I mean safety for you. Not for the pumpkins.