I asked my husband about the spud guns and he said something to the effect that we'd had an extensive discussion about them and didn't I remember that? Must have slipped my mind.
Edit: I accidentally edited my earlier posting instead of quoting it in this new one as I'd intended. I hate when that happens.
Has he thought about something smaller? Spud guns can be very rewarding on a much smaller scale. He can Google it on the internet Lots of fun. I know that your link shows a catapult, but pneumatic or ignition systems can hurl veggies, too.