Just be sure that you're not dry firing (no pellet) a spring piston air gun, cuz that's a quick way to destroy it. And i don't think the blast from an air gun could be felt with much force from 20+ feet away. This is a little different as it creates a swirling donut vortex of air that retains alot of enery as it travels across a room.
MonkeyK, My diaphram sinks in a good 1.5". So the total volume of air is greater with mine. Try increasing it.
That's a good site, but doesn't go too much detail about dimensions and stuff.
Anyhoo, I made my full size Airzooka, and man is this thing strong. It seems everybit as powerful as the real deal (judging from the videos found
here). I shot out a candle at about 20 feet, and think i can go farther. But I have no handle for now, so it's a pain to hold.
-patchy