I discovered this a little while ago, and I immediately wanted one. Until I saw the price. I didn't want to pay $40.00 plus shipping for something I could easily make myself. So I got to work on my own, and I am very happy with the result. I built it for a tenth of the cost and was able to choose my own shape and look for the gun.
Supplies - Toy gun - Duct tape - Wire - Battery pack - Superglue - Motor - Momentary switch - Pencil eraser - The shaft of a multi-bit screwdriver OR an extension bit - Forward/Reverse switch (if you have one skip step 2) OR if you don't have one: - 2 small nails - 2 CONDUCTIVE magnets - Magnetic tape - A 2x2 Lego brick Tools - Hot glue gun - Rotary tool - Soldering iron and solder - Screwdriver - Drill