This is a poorly done instructable but the outcome is great! I will make up for the lack of step by step pictures with a very detailed set of instructions and parts list, I made the whole thing for less than $10.00 US. Money well spent!

Step 1: Fill Valve

The fill valve is installed in the flattest part of the end cap to get a good seal. I used 2 part marine grade epoxy on the inside to seal it as opposed to super glue. It has held for quite some time and seems to be totally air tight. It passed the bubble test, you should put light pressure in the chamber and apply soapy water to all the joints to see if you have any leaks. If there is leaks you will see tiny bubbles.

Step 2: Valve Side of the Assembly

This is pretty self explanatory by mousing over the picture. I used this combination for a number of reasons, cost and parts on hand played a major part. I am planning on building a new one with a "tee" before the valve and putting in a 1/4inch fit in bushing and attaching an air hose fitting. This will allow me to attach the air hose from the compressor, thus filling the chamber to the pre set amount of air everytime I close the valve! It should work...
I will also take pictures step by step on the next instructable!
I am also planning to make another air cannon out of inch and a half copper, choked down to half inch copper to use with potatoes and other fruits and vegtables...

Step 3: More Details on the Valve.

I can't stress enough that you do NOT need an expensive electric valve. Try using the cheap manual valve, if that is not good then move up to the expensive one!
I also prefer to use threaded parts whenever possible, this prevents weak glue joints and if you didn't notice the walls are thicker on the threaded ones. I also like the idea that I can change the parts out easily, If you use the glue on type, there is only one way to remove them, CUT IT OFF! Do NOT try to re-use glue on fittings, It is dangerous!

Step 4: Time for the Air!

First off, you can add air within hours, BUT to be safe, wait at least 24 hours! It can blow the glued joints right off it is not completely dry. Also make sure that the part are fully inserted into the fittings. You must also remember to primer everything.
Contrary to popular belief, this is an important step! Apply the primer and let it dry to the touch (usually a few seconds) Then apply the proper glue to both pieces. when the part is fully seated give it a 1/4 turn and hold it 30 seconds or it will pop out. the reason you give it the 1/4 turn is to make sure the glue is covering the entire surface of the fitting. Color of the primer is not important as long as you use PVC primer. Clear is not a good choice only because you can't see it! You want to make sure the entire area to be glued is properly primered.

Now again, when you add the air start of with a low pressure, there is no need to fill it up to maximum capacity, It is not a safe practice as you do not know what the max capacity actually is! You can't go by what the pipe is rated at because you don't know what the glue joints are capable of holding safely. I have not blown the end off the pipe yet but I can only imagine what that would feel like!
Lastly, DO NOT POINT THIS AT ANYONE FOR ANY REASON! No matter what you have in the barrel! It can cause serious bodily harm!
I am not responsible for any injury or damages that may occur from the use or misuse of this device! I do not recomend building or using this. It is purely for educational purposes!

Step 5: Make Your Child or Yourself Smile From Ear to Ear!

I know I used the same picture several times. I was to excited to take pictures throughout the process! I am new to this sight or any sight for that matter! I have never posted an instructable, so I am open to comments or criticisms. Please keep in mind that I am fully aware that this has been done before, however I think I improved on at least one item, the fill valve, No super glue! The epoxy should hold forever!

As I stated somewhere earlier, I will use a threaded truck valve stem on the next one. This should eliminate the need for epoxys all together.
I would also recomend using a clean out cap on the end, this is the screw on cap. I think it would be better because it would provide a flat surface for the valve stem for one thing and would also allow you to replace the vale stem if needed.
I also think putting the valve stem on a "TEE" fitting would be a good idea so that the valve stem would be on the side as opposed to the bottom.

I did not do an over under design only because I felt that the bends would slow down the release of air. It may be trivial but I may be right?
I have a million ideas that I can improve on this, I learned through trial and error!
I also think this air powered one is as powerfull as my pyro spud gun or even more powerfull! It is definatly more predictable, you open the valve and it fires! As opposed to click click click boom etc.
I will stop talking now! Please let me know if you have any ideas to improve this or ideas for a new one. I have tons of plumbing parts laying around my garage! I also have plenty of free time and have the mentality of a 10 year old! No offense to any 10 year olds out their! Some of the best ideas come from kids, sadly they usually don't have the cash to build them, however I do and am willing to build these toys within reason! I won't do anything dangerous or harmfull though.
I think I will do the Mini-Mag light laser soon, I just think I will make a few little changes to it!
Thanks in advance for your comments!