What the heck is this? If you know anything about gas or plumbing...

[crashwg]

I'm tring to construct pretty much the same thing that is in the picture and I'm at a loss as to what the thing with the guage is.

I know it's some sort of pressure regulator, but I'm not quite sure what it's normally used for or where to get one.

If you're wondering what it's used for, you can go here - http://www.spudtech.com/detail.asp?id=14

BTW, please don't lecture me in the danger of messing with stuff like this.  I'm an adult and I will deal with the negative effects, if any happen to occur.
If you would rather not give me advice on this in fear that you will be held responsable in any way, just don't post anything.  Nothing would bother me more than to know you know but just won't tell me...  

Anyways, here's the picture

[Tailgunner]

Looks like a regular compressed air regulator. Lowes/Home Depot both sell them in the tools section.

Given the choice, I'd build a dump valve compressed air cannon. They're much more effective.

Only words of caution I'll offer: PVC shrapnel doesn't show up on an x-ray.

[Brax]

It's an air regulator with gauge. You can buy better quality brass ones for 5 or 6 bucks at any store that sells airtools.
http://www.mytoolstore.com/astro/astmat10.html

That design is a little suspect. ESPECIALLY since all you fuel metering and igniter are mounted ON the cannon! Duh! Here's a site that shows the end results of multiple firings...
http://www.sentex.net/%7Emwandel/cannon/cannon.html