First of all have you fired this gun before and how is it built there are a variety of ways and orientations to build a spud cannon. Fuel I use is AXE Deodorant a few quick squirts seems to work best as you can over load the cumbustion chamber. Now thats on a 3 inch by 32 inch cumbustion chamber reduced to a 1 1/2 inch by 42 inch long barrel. It fires approx. 300 meters relatively accurately, not sure of the FPS ( speed ). The same Cannon has a second interchangable barrel that is 48 inches long by 3/4 inch. That barrel takes the same ammount of fuel but fires much faster and approximately 500 meters. Lastly, yes you can fire from the shoulder like a rocket launcher if it is designed to. I have made a turret style cannon with 6 barrels, 2 at 1 1/2 inches by 42 inches, 2 at 1 inch by 36 inches and 2 at 1/2 inch by 32 inches long and it mounts over the shoulder with two handles one on each side with 6 trigers. firing them all at once has a bit of "recoil" or kick but a single barrel launcher shouldn't have enough to worry about I let my 6 year old niece fire my single barrel mentioned in the beginning with just enough kick to knock her down. Now if you have a cannon with a 12 inch chamber reduced to a 2 inch chamber I would jold on for dear life but other than that you should be fine.
Hope that helps... And here build one of these:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&item=170068096487&ssPageName=STRK:MESE:IT&ih=007
XtremeEagle