I clicked your link but I got the typical yahoo/geocities "page not available" message so I don't know what you have in mind. It's not a railgun is it?
But almost every day, I see teenagers in our playground launching water balloons about that far with a large slingshot. It's usually a few boys trying to hit the girls to get their attention. This slingshot has thick rubber bands about 4ft long each side with a large pocket in middle of back and a heavy loop at each end. Two guys hold the looped ends, one on each side, by stretching an arm straight up while holding the loop, so they make the arms of the slingshot. A third guy puts a water balloon, usually about 6" to 8" diameter, into the pocket and pulls back as far as he can or as required by the mathematical calculation of trajectory and target location. I've seen these balloons fly over 100yds since the boys like to hide inconspicuoulsy on the far side of the playground and lauch balloons over the football field over the tennis courts, onto the basketball courts where the target girls get splattered. Now that I think of it, their screams and giggles are exciting in a tilted kind of way
If you need more high tech and more distance, maybe we should study methods used for pumpkin chuckin. There's an annual contest in Delaware State, USA where pumpkins are shot as far as possible and I think the record is over 5000ft. The launchers fall into classes like pneumatically powered, steam powered, spring powered, catapault types, etc. Check out this link if you want to launch balloons from San Diego to NYC ...please aim a bit northeast since I'm near Philadelphia PA.
http://www.worldchampionshippunkinchunkin.com/