My friend Christian Ristow was at Maker Faire with his giant pneumatically powered sculpture called Hand of Man. It's great. It's a highly interactive piece in which one puts on a glove with sensors and controls a multi-ton pneumatic hand capable of picking up and crushing a refrigerator.
About a year or so ago, I worked on a TV pilot for Discovery Channel starring Christian. He is perhaps the most gifted mechanical artist I've ever met.
Ristow designed a machine gun potato cannon which was a true machine gun spud gun. It had a gravity fed magazine that fed spuds into the firing chamber. I've built a lot of spud guns in my time, mostly like those in Backyard Ballistics. This was a magnitude more powerful and complex. There were four high pressure air tanks that could shoot potatoes continually and at high velocity until the magazine was emptied. I dubbed it "the Quadra-tater."
The airtanks were massive. I calculated the muzzle velocity to be well in excess of 85 mph. The rate of fire depended on the speed with which you turned a crank. The crank controlled five pneumatic solenoid valves, one for the magazine loader and one for each of the air tanks.
It worked absolutely great. We could get 20 or potatoes in the magazine and could empty the thing in much less than a minute. For the finale, the Quadratater, along with a gatling gun that Dave Mathews built, destroyed a car.
more Quadra-Tator images on my blog at Notes From the Technology Underground