Okay, yeah, it's not my 'first' actually, but it's the first one that's been finished to operational standards, instead of just going to 'proof of concept' and no further. 
After building a small hammer valve for a project I'm working on (airsoft shotgun) I thought to myself, "Hey, I LIKE hammer valves!!" They're elegantly simple, remarkably efficient compared to ball valves, and much cheaper than QEVs. And they're easy to make. Since I had a couple of fire extinguishers lying around, I figured I'd make an airsoft heavy weapon for our frequent games. 8)
It started out as just the hammer valve and air tank, with no particular design intended. I was thinking about using it to make a shoulder mounted RPG type thing, but then I figured it might be more fun and more fair to build it into a heavy artillery weapon. And darned if it isn't just that!!!
I test fired it at %40 of the planned air pressure with a shotgun style sabot with around 25-50 BBs in it. Ended up with a six foot BB blast that went about 60 feet. Not massive range, but that's not the intent. At the full power (planned 100 PSI), it should be firing around 100-150 feet with a much bigger spread. Perfect for war games. 
What's that? You want pics? Okay, here's some to get ya started.
