This is a 3/4 inch pneumatic cannon that is very simple and cheap to make.
THE BEST AMMO FOR THIS CANNON IS PAPER DARTS. they are even simpler and cheaper to make than the cannon. I cannot take credit for this because i found this from someone else but i will put a link for it.Paper Darts

P.S. yes, i did use a few images a couple times.

Step 1: Materials

2- 90 degree elbows

1- long piece of 2/4 inch PVC piping, buy enough for your standards; longer or less depends on you

1- ball and socket valve 3/4 inch

1- 3/4 inch end cap

1- tire valve replacement from autozone or pepboys etc.

1- can of PVC cement
Tools:
Drill
hacksaw or radial arm saw
sandpaper
tape measure
bike pump WITH PRESSURE GAUGE
pen

DISREGARD THE PIECE THAT IS CONNECTED, THE TWO ELBOWS AND SPLIT PORT, it was for an add on but i messed it up =/

Step 2: Measure and Cut the Pieces

measure how much piping you have and make a smaller barrel than the compression chamber.

measure the height from where it is comfortable to hold the barrel at to fire to the top of your shoulder so you can shoulder mount this.

cut the barrel long enough but don't overkill this.

leave the rest for the compression chamber.

Step 3: Cement!

add a generous amount of PVC cement to the inside of the elbows.

glue all the pieces together and push together with a lot of force for about 2 or 3 seconds to get a good hold.

DO NOT CEMENT THE END CAP YET! IT STILL NEEDS THE TIRE VALVE!

Step 4: Drill!

Grab your drill and drill bits.

you will need a bit that is slightly bigger than the bore of most of the tire valve so it sticks out but gets the other end caught.

drill the hole and make sure the tire valve is facing out the exit side of the end cap and clears it.

super glue the valve into place. (I've tried to glue it on the inside but it's very difficult and glueing just the outside works just the same.)

Step 5: Cement Again

AFTER THE SUPER GLUE HAS SET, cement the end cap to the end of the the compression chamber.

you might have to let the cement sit for quite a few hours for the best hold.

(i've blown the end of the end cap off of the cannon before and it will cause damage so be carefull!)

Step 6: Test

pump up the cannon with the ball valve shut. a bike pump with a gauge works better than a compressor (sounds stupid but trust me).

find your ammo wether you decided to use the darts or not and test fire it!

you'll love the reactions of neighbors.

(not advised but i shot my friend with a dart at a very low PSI and it hurt him much like an airsoft BB.

I'm making a second instructable on another cannon that is a better bore size and a little more complicated later.

these can reach quite a bit of distance, such as about 100 yards or so.

finally i would suggest cardstock paper for the darts because at over 40-60 PSI they tend to break and spin out of control.

have fun and don't kill anyone.