Next to the question i wrote i will make an air cannon so there it is. We will need a lot of tings :

materials26 inch long 1/2 inch pvc pipe
2 1/2 45 degrees elbow
11/2 90 degrees elbow
1 1/2 tee piece
optional 3 1/2 end caps
1 3/4 metal ball valve
1 1/2 tred connector
1 1/2 by 3/4 female tred metal connector
1 3/4 tred tubing connector
1 1/2 male coupler
epoxy
a cheap cone of plastic thats fit in the tee

tools
hacksaw
tootpick
sandpaper
teflon tape

Step 1: Epoxy Time

The first step very simple but it can get tricky.
You only need to glue a 45 degrees elbow to a 90 degrees elbow like on the picture.

Step 2: Cut,cut Cut

Take your 26 inches long pvc pipe and to cut 3 5 inches long pieces,1 8 inches long,1 2 inches long and 2 1 inches long.

Step 3: Assemble the Tripod

Take the piece made out of two elbow and put the 5 inches pieces in the 3 bottoms holes and do the same ting with the end caps.
hint: Make 1 4 inches long piece and 2 5 inches long.
It will be pointing up.

Step 4: Tripod Top

Take the 2 inches long pieces and sand the first inch.
It got to turn easly in the tee .

Step 5: Assembly

Now take the ball valve and connect it to the tred connectors and do like on the picture.

Step 6: How to Fire

The first step is to connect the air cannon to an air tank(made out of pvc pipes).
The next step is to put air in the tank with a bicycle pump(10 psi to 40 psi).
The last step to fire is to pull the valve very fast.
For the projectiles i use mini mashmalow or a nerf suction cup bullets.

Be safe and have fun