Here is what I came up with:
The air chamber is 2" PVC schedule 40 cut down to about 16 1/2". To each end of the PVC pipe is glued a 2" coupler. I use the coupler that has a ridge around the inside circumference at the mid point. There is a coupler called a repair coupler that lacks this ridge. To the top coupler is inserted and glued a 2" to 3/4" threaded reducer and into the bottom coupler is inserted glued a 2" flat plug into which I installed a tire valve BEFORE I glued it in. The tire valve is a little bigger than 3/8" diameter so I used a 3/8" bit and rounded the hole out until the valve just barely fit.
The trigger is a 3/8" PVC ball valve and that is connected to the air chamber by a 2" long 3/4" threaded riser from the sprinkler section. The barrel is also a 18" riser from the sprinkler section. The mess is glued together with HOT BLUE glue and left to set up for 24 hours. The 2" riser screws between the ball valve and the 2" to 3/4" reducer on the top end of the air chamber. The threads were wrapped with enough Teflon tape to give an air tight fit. The barrel joint was left un-taped as I unscrew this to place the wadding and to retrieve the line once the cannon has fired.
Here it is all glued and screwed together:
Most people use some kind of fishing reel but I have found a reel as is pictured works very well and is dirt cheap (as in free). I made it from a pop bottle (actually a vitamin water bottle). I cut a hole in the base a little smaller than the barrel and then cut little radial slots into the plastic so the bottle would press fit tightly onto the barrel. Into the cap a hole was cut just big enough so the cap would screw onto the barrel threads. This makes the bottle immediately removable but still very secure. Since the barrel unscrews from the body it's ideal to re-wind the fishing line post firing. Here is a close up of the barrel/bottle. Note the rubber band that keeps the line from unwinding when not in use.
I use 50lb test woven dacron line, very strong and very light. For a bullet I use a 2oz egg sinker which is a close fit to the 3/4 inch barrel. I use some cotton wadding made from some bandage material wrapped with tape. I tied the wadding to reduce friction but still provide a tight fit. I used the blow method to determine the correct resistance. I put the wadding in the bottom of the barrel and blew it out using my mouth and lungs. It's pretty easy to get a good trade off between tightness and friction using this method.
The wadding provides a very light weight gasket so the pressure is better applied to accelerate the bullet. I tried to use tape to make the bullet fit more snugly but found wadding works better than tape. Here is a shot of the wadding inserted in the base of the barrel
I use a loop tied in the line of the sinker and a loop in the main line and join them with a big paper clip. The clip is strong, makes the line and the weight easy to separate when I want to pull a rope back over the tree, and has very low mass.
Be very careful with this rig! A 2 oz sinker will easily penetrate a window, like the sun roof of your car sitting in the driveway or it will easily dent the trunk Know where you bullet may land. I have read PVC has no rating official rating for any gas pressure but I also saw a reference 180 psi as a max. I pump my cannon up to about 50 psi using a hand tire pump with a pressure gauge. It takes about 20 pumps.
I tried it out shooting at a 45 degree and a near 90 degree angle to the ground. At 50 psi I can get about 90 - 100 ft height. We have a tropical storm directly off my east coast so I've had to test this during lulls in feeder band rain showers.
When the storm passes I will try passing some lines over the trees.
This is the cheapest cannon I've yet seen. The PVC parts were under $20 at Home Depot. The tire valves were $5 for a pack of 2 at Discount Auto. The line was $13 and the sinkers were $5 for 5 at the Walmart. I already had the glue and the teflon tape but that might add another $8 or so if you didn't have that.
2 ft 2" cut PVC
2 x 2' couplers
1 2" to 3/4 inch threaded reducer
1 flat end cap that fits into a 2" PVC coupler
1 2" riser 3/4" threaded (sprinkler section)
1 24"long 3/4" threaded riser (sprinkler section)
3/4" PVC threaded ball valve.
Wide mouth pop bottle
150 yards dacron 50 lb line
2oz egg shaped fish sinker
wadding made from cotton bandage material.
All of this is made from materials available anywhere, so it's easy to make your own.