A QUICK WORD OF WARNING, PLASTIC IS NOT THE STRONGEST MATERIAL TO MAKE A CANNON OUT OF AND IF YOU FORM AN IMPROPER BACK SEAL THE REAR OF THE CANNON CAN EXPLODE OUT. PLEASE USE WITH CAUTION, ONLY USE SMALL AMOUNTS OF POWDER, AND USE PROPER PROTECTION AND A BRICK OR WALL AROUND THE CANNON DURRING THE FIRST FEW FIRINGS.
Step 1: Materials
1. bic pen
3. small sharp metal tool with small diameter to punch out the fuse hole like the red one pictured below, but a paper clip will work just as well
4. a small knife to help shape the plastic when its too hot to touch
5. a small thin stcik to use for measurement purposes, just make sure it will fit inside the pen an dis longer than the total length of the pen.
Step 2: Sealing the Pen
1. Start by taking apart the pen. Remove the cap, the ink, and the end that holds the cap on. Leave the end cap on the other side inplace, it helps later on.
2. Start by heating the end of the pen with the end cap still in place, focusing the flame a few centimeters from where the end cap ends inside the pen. Keep the flame low enough below the pen to keep it from catching on fire, it's not the end of the world if it does but it does give off some nasty gasses if the plastic burns instead of melting.
3. Once the plastic is thouroughly heated and begins to look malleable, carefully grab the very end of the pen and twist slowly while pressing it into the body of the pen. keep twisting. now you can either twist off the end entirely and flatten the end like i did, or you can simple twist the end enough that it is sealed off but still attached to the pen. It is your choice as I am not sure which is stronger.