Step 1: Ammunition
There are two types of pellet that I use:
Ball: This is a small made of model magic that is slightly smaller that the diameter of the barrel. It has to be small enough to roll easily down the barrel, but big enough to keep air from flowing past too easily.
Bullet: This is a bullet shaped pellet that is about the same size as the barrel so that it wedges in. It needs to be between 3/4 of an inch and an inch long(any longer and it will go end over end in the air and decrease accuracy and range, any shorter and it will fip in the barrel and jam). It is better with a pointed end.
Don't use standard modeling clay, as it is too heavy and it will stick to the barrel. You can use types of clay that harden, but they are very hard and dangerous, so be careful.
Try experimenting with other shapes and let me know if you find a good one.
Step 2: Pieces Needed
You need the following lengths:
1 15-16" piece
2 6-8" pieces
2 5-6" pieces
You will need 1/2 inch CPVC couplings for this gun.
This gun needs 2 T couplings and 2 end caps, but I suggest buying some extras. It is good to have extra Ts and endcaps, and 90 and 45 degree elbows and straight couplings are fun to experiment with.
The last needed item is Model Magic for ammo.
Step 3: "Bolt-Action"
The secret is that one of the 5-6 inch pieces of pipe has a cut in it 3/4 of an inch from the end almost to the bottom, and across to the end. This leaves a small ledge that will block the hole for the back handle/magazine when the handle is down. This pipe will not fit into the T properly, so you will need to whittle the bottom of the ledge down to fit it in.
The way that this works is fairly simple. when the handle, which is full of balls, is turned upward it is clear of the ledge, and a ball falls into the barrel. When the handle is turned back down the ledge isolates 1 or 2 balls in the barrel and prevents them from falling back into the handle. This system will not jam easily because the ammo is round and isn't sticky, and the constant motion of the handle moves the ammo around.
Step 4: Barrel Assembly
Step 5: Handles/Magazines
The front handle can be filled with balls for automatic fire, and the back handle can be filled for bolt-action fire.
Step 6: Fire!
To fire automatically flip the front handle upside down and leave it blow hard to make sure all come out at full speed.
When using bullet shaped pellets you cannot put them straight in the back of the gun, because it will get stuck on the bolt action piece. You can either separate the handle at the T and load there, or turn the the bolt action piece around so that the ledge is in the front T. These pellets go farther and are more accurate than balls, but are a pain to make and to reload, so use sparingly.
Be careful and have fun!