Step 1: Selecting and purchasing parts:
The parts are as follows:
1x Pipe for barrel
1x Reducer (optional)
1x Repair Coupling
1x End cap
For my mini apple cannon, I used 1/2 inch pipe for the entire thing. (No need for a reducer eh?)
For my bigger one, I used 1 inch pipe for everything except the barrel, which was 1/2 inch. I also needed a reducer, so I got a 1X1/2 inch reducer bushing.
Thats it for parts; now you see why I say its simple.
Because of its simplicity, you can use virtually any size parts. However, since this is hand-powered, I recommend you stay on the small end. I wouldn't go above 2 inch. The repair couplings start getting sort of hard to push at larger sizes. Though if you find a good lubricant, they could potentially work.