Use frozen carbon dioxide (dry ice), mix it with water and pipe the steam into a PET bottle. When the bottle at roughly 22bar explodes you can shoot for example a toy chicken over more than 80 meters (We had serious problems to build a projectile that is not destroyed by the power of the launch). We built the device for a creativity competition at our university. And well, we could not place an object accurate on 20m but for shure we did it with the biggest impact. Music on Amazon and iTunes If you are curious, here some detail about the construction: For us secure handling was not only an option. We have seen some people that put water and dry-ice together into an bottle close it and put it into a pipe. But you have good chances that it will explode in your hands. So we put an PET bottle into our pipe and connect it with an CNC made adapter to the presure reservoir outside. Beside that, all parts are made from steel and welded. We experienced that a 1.5l coke bottle (actually we used...
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The only thing I would suggest is something to set the type of tags written to mp3 files. iTunes (among others) chokes on v2.4 which is the default setting for Picard. (Picard writes valid tags but many others do not.)