AIR POWER!
Caution is recommended when using air fired products.
We recommend whenever using compressed air that you use qaulity valves, connectors, and pnuematic cylinders.  I know Home Haunters are on a budget, but a life is worth the extra money.
There are plenty of walk throughs on how to build one of these bad boys, so I will just cover the basics.

What you need:
   - An air tank
   - A solenoid valve
   - A trigger
   - And maybe some tubing

This is really simple to do. I used a storage tank from Menard's ($15) and some pipe fittings to get past the body of the tank and then added an ASCO Solenoid valve (List is $95, but can find on eBay for around $10).  The Solenoid in this instance is just wired to an on/off switch.

The entire bundle (using eBay) can be built for under $50, whereas they are sold at haunt shows for $400 to $600!!!!

To add the machine gun effect we purchased a circuit board from a Haunt Show vendor and later found the board from the manufacture for a third of the value.  I will post the manufacturer's link when I find it.  The board would cost you around $20 and you will need a solid state relay to switch the power to the solenoid ($10 or so).

There ya go, now your propr has the racheting machine gun affect for around $80.  Add a pressure pad (McMaster Carr Tapeswitch) for around $30 and you have a full blown prop for under $125.
  Ok...ok....Air is fun, but what happens when you add water.  Check out the video where we made a water cannon.  Only the green cup as the water in it.

My daughter is seen in this clip holding the cup.  PLEASE NOTE: This prop was tested over 20 times before I let her hold the cup.

Click Here for the video Link!!!
*All images are thumbnailed, please click on any image to see the large size image.