I was in the progress of making a home made paintball gun, but the practicalities of it were just stupid. So with a little modification i made a much better BB gun!
Warning: This thing is VERY powerful even with a moderate pressure of 50PSI.
Step 1: Make the gun/barrel
Amazingly i found a PVC component that screws exactly onto the spraygun.
The next component is a piece of 21.5mm PVC pipe with a garden hose quick connect thing glue inside.
The alu tube of 6mm or close as possible internal diameter (the closer the more powerful) friction fits with a piece of sticky tape inside of the quick connector.
The length of the barrel has a big effect, the longer the more accurate your BB will travel but at a risk of slowing it down over the length and the shorter it is the faster it will travel withing reason.
Step 2: High pressure tank
If you can afford to, rather get the brass connectors they can hold alot more pressure without leaking, and you will have a much more reliable system.