Paraphernalia - Homemade crackers

[klazzykoo882]

My buddy has some chargers and was curious exactly how a cracker works. From what he understands the charger is held in a some kind of metal/plastic tube snuggly, covered with another piece to seal it air tight and lead the no2 out in a given direction into a balloon but how is the charger "opened". He read they use nails in homemade ones but this seems kind of dangerous combining high pressure and nails. Hes not sure how they are "cracked" either with ones found on ebay and such.

Anyone know someone who tried making their own and could give any advice or blueprints so my buddy could maybe design his own. And would it be better to use brass, copper, pvc etc because of the extreme cold.