First piston rifle (scaled massive egg cannon)

[noob of noobs]

Okay, so my amigo and I are building an egg cannon. The chamber is gonna be about 5 feet of 3" PVC pipe, the barrel about 4.5 feet of 2" pvc with a 3" piston valve piloted by a sprinkler valve. Problem is, I've never built a piston valve gun before, and want a little experience before we commit all the money into this massive gun (for my standards, yes, this is massive.) So, I decided to make a somewhat scale model with a blowgun pilot as the first experiment. Here’s the picture:
http://img402.imageshack.us/my.php?imag ... gunhg5.png
It's a bit unclear on immage shack, but if you copy the picture and paste it into microsoft word, you can stretch it out and view it to full size.

Now, I'd like to get bang for my buck, and I want this to be a good gun, preferably a sniper. As a noob, I got a few questions that I couldn't find answers to online.
1. On the end of the T piston with the pilot and schrader, do you have to modify the plug? Looking at a .5" plug in a .5" tee, it doesn't look like there'll be much room for the piston because the plug end is blocking it. But I know I've seen people put the plug directly into the T, so it is possible.
2. I know I've heard that incorporating a stock into the pressure chamber lowers performance because of all the bends, but will my design be okay? On both sides of the T leading to the piston valve, there's a pressure chamber, so as long as air is coming in from the primary and large side unhampered, then would a secondary pressure section interfere? It isn't obstructing air from the regular tank, and in theory, it should improve performance.
3. As you know, I'm making a scale model of a 2:3 ratio cannon. Should my scale model be a .5" barrel and .75" chamber, or a 1" barrel and 1.5" chamber? Also, will a 2:3 ratio be adequate to hurl a saboted egg far? Does a 1:2 ratio sound better? (The actual egg cannon is supposed to be portable enough to lash on your back by the way, and I only have a foot pump to fill it)
4. Does a sprinkler valve on the big cannon seem like overkill? My budget is a bit slim , so if possible, I'd like to try to avoid one...
5. Will a coupler at the end of the barrel help mute the bang, and will it improve accuracy?
6. I know I saw a piston design that looks like an axle with wheels on it, but can't find it on the forums... It seems like it'd be lighter and better, but I just can't find it. Does anyone know where to find the design, and are they better than traditional solid pistons?
7. Is a bumper necessary on soft hot glue pistons? I know it's a must on PVC pistons, but hot glue seems like it'd be soft enough.
8. Are there any flaws in the design?

Thanks a lot! Any feed back, both positive and (cruel,) is appreciated. Thanks for your time.

[singularity]

1) dont house the piston in a 1/2 tee it will be too small go with atleast a 1" tee or bigger. but if by modify you mean drill two holes in then yes
2)you should be fine with your design. in most cases the stock is made from smaller diameter pipe then the chamber and some sort of odd design, but yours is very simple and the same diameter.
3)go with a 1" barrel and a 1.5" chamber it will make it simpler. and for the ratio question i am lost is that a chamber to barrel ratio? cause it doesn't seem right after looking at that diagram, and good luck lashing it on your back that thing is gonna be real heavy
4)i have a sprinkler valve on my 3" coaxial and i love it becuase it gives it a "trigger" the only other cheaper option would be a ball valve and those are slow and annoying, just get the sprinkler valve you'll be glad you did
5)no and no well at least not a noticeable amount maybe .5db and i doubt it will do anything for accuracy at all
6) are you talking about the one that uses rubber sandwiched between two washers?
7)just put a spring in there and you'll be fine doesn't need to be all that strong
8)the only flaw i can think of is it might be hard to find a pressure rated 3" tee but other than that i cant see anything wrong with it. and if your going to make this a sniper i better see a scope on it some where im getting tired of clicking on post that say "sniper cannon" and not find a scope or any form of sight altogether, im going to show you guys what a real sniper is soon

ps- are these eggs going to have lead cores or something cause a massive 5ft chamber seems like overkill for an egg, not that theres anything wrong with over kill

[noob of noobs]

Thanks a bunch, I'll probably pick up parts today. Yes, that's the chamber:barrel ratio is 3:2, I just drew a really crappy diagram. I have lashed a 7 foot over/under on my back with a 1.5" chamber and 1.25" barrel with sprinkler valve before. Some cop pulled me over though. Yes, the piston design with rubber between 2 washers seems like it. Know where it is?
I'll try to get a scope depending on how far this shoots, and I look forward to seeing your sniper. The actual egg cannon is supposed to be more like an artillery piece if you know what I mean, but I think I might cut it down in size a bit for portability and practicality.
Thanks again...

Edit: I'll post it as soon as I'm done.