the project is basicly about adding an ammo indicator to the nerf maverick gun.
connect all parts as seen in the sketch to the teensy 2.0 and the breadboard. upload the code and test. if successful, disassemble the gun (see instructables.com) and place the parts inside the gun. reassemble, enjoy.
you will need plenty of hot glue.
so in the very end, you have to have at least the toggle switch on the outside of the gun.
i drilled a few more holes though. for the mini usb port of the teensy and a power plug for the battery to charge. my battery of choice is a 3,7v lipoly.
PS: recheck the pins given in the fritzing sheet and the code. there might be a pin in the picture that actually has a different position in the code. not to worry though, the code is very well commented.