Its nothing special, I used PVC and at the end of the barrel I slipped on a coupling, a couplings ID is bigger than the pipe ID. Noticed I used 2 valves. When you pull the air filler hose off the valve stem its lets out a little air, how much? We notice a big affect on range until I added the valve.
I cut 2 3/8s round solid steel rods about the same length of the barrel, then I welded washers on the ends, then plugged the bolts through the washers into the coupling. You can add a cross member if you like to keep the legs locked together, I did not so I could set up on uneven ground.
Here is a pic of the first one I made. It should give you a basic idea of what I did. This was my idea, not the only way, if there are better ways out there....
Keep in mind when you build this, what is the wadding you want to use? I found for my use, get Dixie Cups, then match your pipe to the cup. (1 1/2 if I recall correctly) The cup will not fit down the pipe, about 90% of the cup will fit in the pipe, take a knife and cut the cup top off while its in the pipe. then put it in the barrel upside down. (like and umbrella, it catches all the air like a parachute to maximize your shot) Cut a second cup, slit the cup in half vertically all the way to the bottom of the cup. Hold the cup together fill with bbs and drop down the barrel and shoot.
The cups seem like a pain, but its easy, once you see what your trying to do you can cut them with scissors, its fast and easy. The ammo box carried pre cut cups.
send me a PM if you need anything