This is an easy pen gun. You do not need anything but a pen.
Step 1: Materials:
You will need:
-A B2P pen (or a pen with a ring that prevents the clicker pieces from coming out)
-A mechanical pencil (or other long ro that will fit though both front and back
Step 2: Glue: (optional)
Hot glue the clicker to the spring (the one you press on) If you do not glue the spring in, it may fire as well and be lost.
The longer the spring the better (but only within about an inch)
Step 3: Loading:
Push the glued piece in first. Then push in the ammunition in with your loading rod. Done