Making your own paint ball gun is quite a long process and takes a certain amount of tool handling but it is a much cheaper alternative to ones that you can buy form the shops. There are different ways of making them but if you have some PVC pipe, a handsaw, glue and some PVS pipe connectors you have nearly everything you need, once you get hold of a gas cannister.
You will need to start with a piece of PVC pipe that is ¾ inch, which you will need to cut to one foot in length and then smooth off with sandpaper. You will need a good sharp handsaw to do this. This is the barrel and you will need to glue a PVC pipe connector to one end. You then need to attach the gas cannister which is what will provide the energy to fire the paint out of the gun. The threaded end of the gun barrel to the gas cannister and then a PVC pipe connector to the other end of the gas valve. You then need to cut another piece if the PVC pipe, this time about 4 inches long. Glue a female end cap on one end. Attach this cap to the gas valve. It may be a good idea to use some pipe sealer to make sure there are no air leaks in the gun. Before use you will need to fill your gun with about 40 pounds of air.
This process may seem quite simple but you do need to be careful using the tools and it is a good idea to only do this if you are used to making this type of product. It would not be worth having a nasty accident or making something which is not safe, just to save a bit of money.