Cool
But this an't my idea.

Step 1: The Gun

It's pretty obvious.
you need...
tiny advil pill bottle
--2 tacks
-pen tube
-bbq ignighter(inside a utility lighter)
-wire
-electric tape

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.