I had made this project many months back so i thought to show you too .
Step 1: Things you need:-
2 - bottles ( you ll need one with its neck and the other just the cap)
a pen ( it should be more than 6mm in diameter so that if u want u can insert a bb too)
a barbecue lighter
a cylinder shape container (it is used as a grip for ur gun)
drill or scissor to make holes