SpudFiles

K, I've designed my own piston cannon and I would like some confirmation as to whether it will work or not, and anything that can be improved.
Note: the blowgun will be attached to the handle. The chamber is 3" pipe and the barrel and stock are 1 1/2". Anything that is dotted lines is inside another pipe.
I really need to know if the blowgun on its own will vent air quickly enough to suck the piston back fast.
Any comments are needed
Thanks

the blow gun will be slow for a 3" chamber

Any suggestions on what to use instead?
I liked the blowgun, cause it it trgger-like therefore, easy to fire and cool and its cheap

Ok, a few comments:
A blowgun, even modded, will have poor flow.
You could make another (small) diaphram/piston valve for use as a pilot.
Your pilot volume is enourmous.
The large forces caused by the piston are going to be bad for your 1.5" sticking into the chamber.
Use reducing bushings, not reducing couplers, in pressure systems. PW reducing couplers are rare in the extreame.

1.I dont know much about all of the many types of pvc fittings, so what's wrong the the reducing couplers?
2.The reason for the 1 1/2" sticking into the pilot was to stop the piston from falling into the tee and getting out of place, and the end cap on the end stop the air from going into the stock (you probably can tell this) So if that isn't going to work, I need some alternative solutions.
2.And to solve the enormous pilot problem, would it work to reduce the chamber to the left of the barrel down to 2" like in the pic below?
Note: the pic got kinda screwed up when i scanned it

1 - There isn't anything inhearently wrong with reducing couplers, it's just that you won't find <i>pressure rated</i>. The construction of the launcher is still pretty much the same if you use reducing bushings, so don't worry about it for now.
2 - The porpose of it is not the topic of discussion; it has no bearing on weather it breaks or not, something that I see as fairly likely. (of course, it very well might not... but I wouldn't risk it!)
3 - 1.5" doesn't fit in 2", so that wouldn't work very well.

Personaly, I'd construct the acctual cannon without the built in handle/stock - a generic co-axial - and then mount it on a stock.
Keeps the pilot volume low, avoids the issue with the 1.5" pipe protrusion, all those good things.

As to the blowgun as a pilot valve, I ran a GGDT simulation of it:
With what I saw as fairly probable dimentions; a .4" vent (roughly a 3/8" ball valve, for instance) gives 300 foot/lbs, compared to 230 with a .2" vent (modded blowgun). This difference gets bigger with lighter ammo, and shorter barrels.
If you want to retain use of the blow gun as a trigger (and still get good energy), you could use a secondary pilot valve, like a 3/4" qev. This would be actuated with the blowgun. These can be purchased from mcmaster.com for roughly 13-20$, on page 883 - I reccomend the 3/4" right angle valve, as it can be used on future cannons as the main valve (they are pretty good on small-bore launchers).
As a further advantage, consistent firing is acheived by using a pilot-opperated pilot valve, as recent testing with sprinkler valves reveiled.