This is a simple pen gun you can make with no special tools in under 5 minutes. It's fun to use and shoots fairly far.
Step 1: Taking the Pen Apart
Just yank on the grip until the whole front end pops off. To get the cap at the back off, either gnaw on the end or use pliers.
Step 2: Taking Apart the Pencil
First, pull or unscrew the front end. Then push down on the eraser and cut the inner cap (shown in black) off. Pull everything apart and you should get something like the fourth picture. Get rid of everything except the piston-like part in the fifth photo.
Step 3: Trimming the Piston
Make a cut roughly where it's marked on the picture. If you're unsure, leave more, not less, as you can trim it down later.
Step 4: Put It Together (part 1)
Cut the rubber band in half, and tape each end to the front of the pen (pictures 1 and 2). Add the piston from the pencil, and put the rubber band over that, so that it looks like picture 3.
Step 5: Put It Together (part 2)
Stack all of the parts like this and squish them together. It will then look like picture 2.
Step 6: Use It!
To use the pen, first take the whole front part out. Load it with something small, like a knex or a wad of paper, pull the eraser back, and let go.
It can also shoot long, thing things, like the ink cartridge or an unfolded hornet. Just take out the part from the mechanical pencil, and use it with just the rubber band.
It even still writes, so it has an easy disguise, as well as being a backup writing implement.