Umm, I dont mean to be the naysayer, seeing as how that is by far the coolest thing Ive seen on this forum to date, but...
Would a patato be dense enough to remain in one piece with that much air pressure hitting it?
Since its a boom acceleration as my fisix teacher likes to call it, it starts at rest and is suddenly hit with all this energy, I would imagine you'd be shooting mashed potatoes out the other end.
Then again, I hope Im wrong.
My friends and I are gonna be putting something similar together using our spare 20oz liquid tanks. We figure 4 of those wired together should get some amazing pressure, and the expansion rate should be safe enough so we wont have to worry about our chosen projectile (still under debate, but the leading idea is the tippman squadbuster).
When we finish it, I'll be sure to post our mortar cannon on here.
WDP: "Our gun is $400 shinier than yours." - Miscue
"Evil Tom Grinns......" - Tom