These are the exact measurements of this potato gun I have built.  This puts it at about 6 1/2 feet tall with one heck of a kick with normal hair spray and potatoes.  I can probably shoot a potato around 250 yards at about a 30 degree angle going against a light breeze.  Man, they fly out of there at some high speeds!  LOTS of fun!!  Just be carful, it flew out of my hands the first time I fired it because I didn't think it would pack such a nice punch!  Very very loud too! 
These are all the pieces I had to buy to make this gun.  It should give you a good idea on how to build it.
If you would like to make a nice handle like this, it's actually pretty easy if you have a drill press or a good drill with a rough round bit.  Just takes a little time.
1)  Find a piece of wood like a 2x4 or something.
2)  Cut it to a nice size handle, however you want it.  Don't worry if it's hard to get your whole hand around it because when you sand the grooves in it, it will feel nice.
3)  Grip the piece of wood how you normaly would and draw lines with a pencil inbetween your fingers.  This is where the gooves go.
4)  If you have a drill press, put one of those bits that looks like a cylinder with sandpaper on it.  Use some rough stuff first to get the grooves started. Make sure the bit is about how round you want the grooves to be.
5)  Now just sand the handle down where you marked where your fingers go.  When it looks good, use a normal sander to make everything all silky smooth.
-If you don't have a drill press, I'm sure you could probably use a normal drill to make the gooves with the sanding bit.  This will make it much more difficult than with a drill press but you should be able to stap it down with a vice or something.
6)  Drill a hole the size of the BBQ ignitor into the front of the grip you just made.  You can do this before or after you make the grooves. 
7)  Stick the BBQ ignitor through the hole and make sure that the wires go in first!
8)  Now fill in the rest of the space inside the hole with hot glue or epoxy to ensure that it doesn't fall out and stays put.
9)  Find another thin piece of wood the same width as the handle so that you can  attach it to the barrel of the gun.
10)  Drill 3 or 4 screws through the thin piece of wood and into the top of the handle.  Make sure it is very strong, it has to hold all that weight.
11)  Use some hose clamps to attach the thin piece of wood to the barrel.  Make sure its tight.
12)  Now just attach the wires to the electrodes (nails or screws) on the chamber!  Your done!
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