Here's a quick demo video:
Step 1: Supplies
Here is the list of what you'll need:
1 Nerf Vulcan
1 Arduino or clone
1 5v Relay Switch
1 NPN Transistor
1 10k ohm Resistor
A Soldering Iron
Step 2: Making the First (and Only) Cut
In order to do this mod, you must open up Nerf Vulcan. It would be a pain to open up the whole gun, so you need to make just one cut. This will allow you to open up just specific region of the gun.
Begin by marking the region of the gun you are going to cut. (See second picture)
Then use what ever method you would like to cut along the line to the spot where both sides of the gun meet. There are many methods of doing this. I just used a saw. Choose you method of cutting, and slice it down the line.
Step 3: Open It Up
There are a few screws that you need remove. Take out all of the screws to the right of the line you cut. Once the screws are removed, you can just take of the top part. It should be fairly straight forward if you did the previous step correctly.
Step 4: Remove the Cover to the On/off Switch
To have access to the on/off switch itself, you are going to need to remove to little orange cap that goes onto it (see picture). This will allow you to see the on and off switch itself, without the thing that makes it look better.
Removing this cover is a remotely barbaric process. There is one accessible screw on the bottom of it, loosening and removing it won't get you very far though. I took out that screw, realized that I couldn't get to the second screw, and decided to just tear the cover off. It may be a bit crude, but it works.
Step 5: Pulling Out the On/off Switch
This process is even more crude. Perfectionists should look away right about now. The on and off switch sits surrounded by plastic, which can be just cut away. Scissors don't normally work, so I use a sharp, sturdy knife. All you need to do for this step is to remove the switch from its place and keep it exposed.
Step 6: Setting Up Your Arduino
For this, you should have a 5v relay switch that is controlled by pin 13 of your Arduino, and you can set it up really however you would like. You could try to use my method just by looking at the picture, but I will unfortunately assume that you know how to use a relay with your Arduino.
Step 7: Remove the Switch
Remember that switch you worked so hard to expose? Yeah, we're hacking it off. You should see two wires soldered onto the bottom of the switch. You should cut those right at the top in order keep as much wire as possible exposed. If you cut it and you don't think you have a good amount of wire exposed, don't panic, you can just use a wire stripper on it.
I'm afraid I forgot to take a picture for this one!
Step 8: Soldering
This is the only soldering you are going to need to do. You are to solder the two loose wires on the Arduino (view picture two) to the two exposed wires on the Nerf gun. This is probably the most important step in that it it where the Arduino and the Nerf gun finally come together. The Arduino wires can be soldered to any of the Nerf gun wires.
Remember that the wires on the Nerf gun can be stripped if necessary.
Step 9: Duct Tape the Trigger Shut
This one of the easiest steps. All you need to do is, as mentioned, duct tape the trigger shut. This will make it so the relay (the on/off switch essentially) is in complete control over the gun.
Step 10: Load a Program, and Shoot All the Things!
You are now finished! The only thing left is to program it. You can make it do whatever you want; writing HIGH to pin 13 is all you need to do to shoot it. A good test is to use the example "Blink" program, usually everyone's first Arduino program, and change "delay(1000)" to "delay(2500)".
Remember, if you have any question or suggestions, you can leave them in the comments and I will get back to you right away!