Well this is my instructable for my coaxial airgun that I put together.
It costs about $15 to construct, and is very fun to mess around with.
It shoots at roughly 600fps+ at 120psi with a .12gram airsoft bb and the excact specifications of the airgun made in this instructable.
And I must state that ***IT IS NOT MY FAULT IF YOU OR OTHERS ARE HURT OR KILLED AS A RESULT OF THIS AIRGUN***
And a couple tips (pretty much follow them or you will die,no joke)
*Never pressurize it over 140psi, even though it says rated to 480psi on the side.
*Never look down the barrel, even when you dont think it is loaded or pressurized
*Never aim at anything you do not intend to destroy
*Do not use this in an airsoft war, this is meant to just be something to mess around with. It will
penetrated human flesh.
*Give the pvc cement and epoxy a full 24 hours to dry, no less.
Make sure you understand all those tips before going to next page.
And if you dont understand how a piston valve works then this
is a helpful link.