THIS IS NOT MEANT TO KILL DUCKS! I go to Oregon State University, and our rivals are the U of O Ducks. That said, here it is! This is a 2" piston with 1.5" porting for shooting golfballs. I built it because I have gone for months without building anything! (Dorm life is not conducive to big projects, as you may have imagined). The only real distinctive parts are the handles, but I still think it's quite an achievement! It cost about 100 dollars, give or take.
Specs:
Valve: 2" with 1 1/2" porting
Pilot Valve: 3/4" QEV
Secondary Pilot Valve: Blowgun
Barrel Size: 1.7"
Barrel Length: 5'
Chamber Size: 2"
Chamber Length: 4'
The handles are made of 3/4" fittings, and are able to unthread from the main body for easy transport. The blowgun is stuck inside a short length of 3/4" pipe, which makes it ergonomic for minigun-style firing

In the pictures, a tire valve is used to fill, since I only have a bike pump for testing. However, when I get a chance to go to the machine shop on campus, I can use that quick connect I bought

The piston is made of threaded rod, with neoprene disks backed by 2" washers.
I plan on having someone who can do calligraphy paint "The Duck Killer" on the chamber in orange.
Pics: