first of all what you need is:
container or bottle
one eigth drill bit
drill a hole in the center of the lid or cap so that a piece of string can get through so i recommend the 1 8th drill bit
Then measure out enough string to math where my finger is then take some wrap and make it so that it goes inside the cap but enough so that you have room to make it come out oh and put string through the center of the wrap to make sure it fits but also feed it through the hole in your cap and tape it at the top so that the string won't fall through.
it should look like this and now put a piece of tape on the bottom of the string
then pour vinegar into your bottle
next make a baking soda ball with kleenex
then put the wrap on the top of the container (cap off) and rest the baking soda ball on it then leave the wrapand put the lid on making sure the string goes through the hole in the wrap
now your done if you made it right when you pull the string a reaction will occour causing it to create c02 gas thus blowing the cap off or exploding the bottle and if its for airsoft pour some bbs in the vinegar with your speed loader now go test it!