SDdan wrote:The tinfoil idea in neat but I will be in a church parking lot and need all the control I can have.
In which case you can use a cascading burst disc design. 'Tis very similar to the already described burst disc design (ie, the "tinfoil idea") but instead of having...
Barrel / Foil / Chamber
Barrel / Foil / VERY Small Chamber / Foil / Chamber
At that point you fill the small chamber to (say) 3/4 of the pressure required to burst the foil. THen you fill the main chamber to full pressure.
To fire the gun, you can now either:
(a) Fill the small chamber to full pressure (can be done very quickly since it's such a small chamber).
(b) Vent the small chamber to zero.
Voila, a burst disc design that can be easily controlled.