A very simple contraption used for launching elastics. Other than your hand, they don't get any simpler than this. This one uses a pencil for the rod. NOTE: This elastic gun will probably not allow you to double up the elastic (for extra power) due to the small size of the end of the pencil.
Difficulty: Very easy
Time: 1 minute
Pencil - (or other long relatively skinny thing like a pen)
Clothes Pin - (or peg or whatever other names people call them)
Tape - (preferably skinny, so if you have duct tape you'll wanna rip it in half or thirds. I used electrician's tape)
Elastic - (for ammo)
Step 1: Taping the Clothes pin to the Pencil
Place the clothes pin on the very end of the pencil, with the part that you press on at the end of the pencil (see picture). It doesn't matter which end you use, but you might want to take a look at it and see if there are any advantages of using one instead of the other. Tape it down. Also you might want to carve a notch into the end of the pencil to help keep the elastic from coming off before you shoot.
Step 2: Taping the Front of the Clothes Pin to the Pencil
Tape the clothes pin to the pencil about a quarter of a centimeter past the spring groove on the top of the clothes pin. If you tape it too far forwards it could make it hard to open. This step is mainly for that purpose, but not too hard. Do it just like I have it and it will prevent the clothes pin from opening too easily. If it opened while you were setting the elastic on the end of the pencil, it could fire backwards and hit your hand - ouch.