But this an't my idea.
Step 1: The gun
It's pretty obvious.
tiny advil pill bottle
-bbq ignighter(inside a utility lighter)
Step 2: Build
Cut the end off the pill bottle(the end without a cap).Stick in the two tacks into the bottle and make it so they almost touck(like a 3mm gap). Take a wired up ignighter and strip the ends and wrap them around each steel part of the tack and push down(if you dont know how to wire the ignighter too bad!).Tape the ignighter down to the bottle(make shure you can still press the botton).Wrap tape to the end of the pen tube so it snugly fits into the cut end of the bottle.Test for a good spark(rewire if it wont spark).
Put a bit of potato in the pen tube. spray a bit of hair spray in the screw end and put on the cap.
fire and enjoy
Ps. this is my 1st instructable tell me what u think.