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 Deit bottles that explode in a better way) can withstand up to 22bar of pressure (what is more than most sources on the internet report)
The next design decision we had to make was a system to quickly inject all the water while not blocking the exhaust of the considerable amout of CO2 gas that is generated as the water mixes with the dry-ice. We first tried it with an single valve but had a problem to get more water it at some point when to much CO2 was exhausted. Howerver two, one for water injection and the other valve for the gas exhaust were ways better.
Finally, the manometer is not necessary but nice to have as it makes the firing a bit less surprising. All in all we spent about 90 Euros.