After observing several different air gun designs, i decided to make my own using cheap materials which anyone could obtain easily. Most DIY air guns consist of a barrel(most likely pvc), a open/close valve(usually a ball valve) and a air storage tank(as well made of pvc). A bike pimp or air compressor is usually used to refill the air tank after 1-3 shots from the gun.
In this version of a DIY air gun, nothing more than a old soda bottle is used for a air tank and a cheap ball inflator is connected directly into the back of the gun for fast air refills in battle(saves a lot of hassle). The whole project consists of only 5 parts and will cost you 7$ at most. It will as well take only 10 minutes of your time if you have all your supplies on hand.

Step 1: Supplies

Like i said, there are only 5 parts to the gun. You will need a 1/2" pvc ball valve(threaded), 1ft of 1/2" pvc pipe, a threaded 1/2" pvc end cap or threaded plug, a soda bottle, & a cheap dollar store ball inflator or bike pump. See Picture.

Step 2: Idea

Ok so the main idea to this gun was that it could be simple, lightweight, and could come apart in two parts for easy transportation, but could still easily shoot a nerf dart 35 feet if it had to. From simply looking at the supplies picture, and then looking at the finished product, you can probably tell how the gun will come together.
-The parts come together in this order-
(Ball inflator)-(bottle)-(threaded end cap)-(ball valve)-(pvc barrel)

Step 3: Firstly

Before placing any of the parts together and gluing them(you will need a lot of hot glue for this project), you will need to drill some holes.
So first using a knife or drill you should drill a tiny hole in the back of the soda bottle, just big enough for the inflator needle from the ball inflator or bike pump to fit through. Next your going to want to drill a hole in the cap of the soda bottle, and into the threaded pvc end cap. The holes should be roughly the same size but it doesn't really matter. Once a hole is drilled in each piece, the two caps should be glued together back to back. You may notice that in the picture a small plastic tube is placed & glued through the hole in each cap. I only did this to make it easier for air to pass from the bottle to the ball valve without any air leaks. You can check if this piece is air tight by blowing into one end while holding the other end shut with your hand. Listen for air leaks.

Step 4: Air Tank

This is an easy step. Just take your bottle and your ball inflator, then push the inflators needle inside the small hole which you drilled in the bottom of the bottle. Lots of hot glue should be put around this connection, not only to make it air tight, but also so that the inflator will stay firmly at the back of the bottle.

Step 5: Valve+barrel

Now take the pvc pipe and the ball valve and connect the two using hot glue. Hot glue is needed because the pipe will not quite fit firmly inside the ball valve. Make sure you glue the barrel on strait. I have as well made a wooden grip for this gun as you can see. All you would need for that is some scrap wood and a hacksaw. And then maybe some sandpaper to smooth it down afterwards. I glued my grip to base of the ball valve but get creative with your gun designs.

Step 6: Piece It Together

You should now have some assorted pieces such as these.

Step 7:

Screw one side of the cap you made into the bottle.

Step 8:

Now screw the other side into the open end of the threaded ball valve.

Step 9: Finished!

Your done from there, feel free to add comments or advice on the design.
-How to fire-
To fire, load a nerf dart,.50 cal paintball,marshmellow, or anything else that fits, down the barrel.
Make sure the valve is closed.
Pump the inflator 3-15 times.
Open ball valve.

Note: 3 pumps will not do you much good, it will send a nerf dart flying no more than 8 feet. Where as 15 pumps will send a nerf dart flying almost 40 feet, depending on the type of dart you using. Have fun! Wear eye protection!

Step 10: Extras

Here is where you can get the supplies and how much they will cost you...

Pvc pipe-home depot-0.50$
Pvc ball valve-home depot-4.00$
Threaded pvc end cap-home depot-0.50$
Soda bottle-recycling bin-free
Ball inflator pump-dollar store-2.00$

How it works:
Knowing how the gun works is just as important as knowing how to make it. So just in case your wondering about how this could ever work, here is how.
When the air pump in the back is pumped, it sends air directly into the bottle, where the air is stored and pressurized after several pumps. Because there is a opening between the bottle and the ball valve, air pressure is able to build behind the valve so that when the valve opens, air pressure is released down the barrel shooting out whatever is blocking its path.

So there you have it, one of the most easy to make air gun designs. Hope you like it:)