Step 2: Building it
The first thing you need to do is take the ridge off of the 2" to 4" couplings. This stops it from sliding onto the pipe and needs to be removed with a dremel tool or sandpaper.
After that is done, put the 2" to 2" coupling onto the end of the 2" pipe. Slide the 2" to 4" coupling down the pipe untill it touches the 2" to 2' coupling. (do not use glue)
You should then decide how long you want the inner barrel to be. i made mine 14 inches long but you make want one larger or smaller depending on the size of the cannon.
After cutting, slide the 4" pipe onto the 2" to 4" coupler and determine where you want to cut the 4" pipe.