Last night I was really bored and scanning my hundreds of channels and came across Punkin' Chunkin" for you guys playing with those tinny tiny 600 OK's.
These guys are building really BIG BORES. They will use compressed air to hurl 8-10 Lb pumpkins 3000-4000'. Spending up to $100K to build their GUNS! Some of the barrels appear to be 30-40' long.

They are smoothbores, not rifles, so this is more appropriate for 'Fun Shooting'.

George

On a smaller scale, check out Backyard Ballistics (the web site) and build a spud gun.

I have built several pneumatic spud guns and charge them with a SCUBA tank and regulator. They seem to be much more effective and efficient than the hairspray powered guns.

The link to Pumpkin Chunking on the Backyard Ballistics site seems to be broken.