sthorne... 
I'm not hammering on him, 

  I'm trying to point him in a right direction and tell him to plan it out before he spends $$ on parts he may not need.  I'm talking from experience, I've done that before when I was just starting.  I'll be the first to help him out, which I thought that's what I was doing.  I wasn't trying to be a smartass and beat on him like alot in here would do.  
Dude, I even posted a 'clue' for him.  By all means, I'm glad he's here.  5000+ members all throwing ideas at each other.... what could be better. 
EDIT:  He may even do as I did and decide not to use a sprinkler valve for the main valve like I did.  I came in here thinking I was going to build a simple cannon using a chamber, sprinkler valve, and barrel.   After reading for a week or so, I decided to build a piston valve instead, using my sprinkler valve as the pilot.  That's what I was tring to get across.