I have some experience in the field of potato cannons. MY signature one can be found here, puncturing a phonebook. The problem with this one is that it is unwieldy (12 feet long at max. length), complicated (many hours of Spudfiles research), expensive ($100-$150), and it takes a while to prep it for shooting (3-5 minutes). In making the apple cannon, I wanted to solve these issues. At the sacrifice of brute force, I believe I have solved these problems. The apple cannon is small enough to fit in an athletics bag to be brought to school with (which has come in use for plugging some fellow ninth graders.) It also needs no peripherals (no pump). I can get off around 3-5 slugs a minute. Finally, it is only made up of 3-4 parts depending on which type you choose to make. All of these parts can be found at any self respecting hardware store.
Step 1: Selecting and Purchasing Parts:
Practically any hardware store has the parts you need.
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.
Step 2: Assembly:
Assembly is really simple.
All you need to do is put the parts together, almost like legos. Just put the barrel into the socket end of the reducer bushing, and the bushing into the socket end of the coupling. Finally, put the end cap on the end of the repair coupling. If you want to, and know how, you can prime and weld the PVC together, but since we are not dealing with huge forces here, the pieces stay together fine without PVC glue. Possibly the hardest part of making this gun is beveling the tip of the barrel. It wasn't very hard on a grinder, but I can imagine it might be sort of time consuming with only a file. Just make sure it is a gradual bevel. Gradual as in more so than the picture. I found that the PVC is a little thick for the apples, so it cracks them, which sometimes results in bad slugs, but most often results in wasted apple. To remedy this, putting a more shallow grade on the bevel would make the PVC thinner at the end, and result in fewer cracked apples. Another tip for the bevel is to put a tiny bit of a bevel on the inside lip. This creates a slug that is slightly over-sized, so it creates a better seal.
Step 3: Shooting!
Now you have your apple cannon, its time to shoot! Like everything else, this is also simple. Just put an apple to the barrel and push on it so the bevel cuts into it. Twist the apple off to the side to break off the slug and leave it in the barrel. !!! make sure the repair coupling is fully extended at this point!!! Use a pre measured rod to ram the slug to the point just before the barrel hits the reducer. Finally, brace the end cap on something, the palm of your hand works well for aiming horizontally, but putting it on the ground and shooting up works best. And then pull back on the repair coupling body, reducing the internal volume.
Caution: This one isn't terribly obvious either...
While loading the apple, make sure you take care that you are applying the force in such a way that if you suddenly go through the apple, or it slips, the bevel does not take a chunk out of you.
Also, don't aim at someone's face.
I must say, this apple cannon is a little finnicky. Due to the nature of its power, (by hand) it is not terribly powerful. The only way to get it to work well is by making a good slug with a good seal. Make sure the bevel is quality work. THIS GUN WILL NOT WORK WITH ANYTHING OTHER THAN AN OBJECT THAT COMPLETELY SEALS THE BARREL AND DOES NOT CREATE A LOT OF FRICTION AGAINST IT. <==this statement is not meant to be angry, just emphasized.