Here is a short video of the awesome Lego cannon:
The robot is build using the pieces that are available in both EV3 kits and is programmed (sadly) in Java on LeJos.
This file contains building instructions of this model and this is the model itself created using the Lego Digital Designer. For those who don't want to read a step by step guide, here is also a brief description:
You need to place the motor that creates the horizontal movement in the bottom, because else a) it also makes a vertical movement depending on the angle of the other motor b) it won't be stable enough. In order to increase stability, you need to construct a table, on which the other motor (connected to the first one and carrying the last one) can glide, because this one carries some weight and is likely not going to stay parallel to the ground without. Lastly, you place the Medium Regulated Motor onto the last one with the shooter. I have used the system from the EV3RSTORM model of LEGO with some minor modifications on connection to the rest of the system.
The source code for the robot is on my github. To get it running, create a Lejos project, add this file to src directory, compile and upload to the robot.
You can control the robot using the arrow keys on the intelligent brick, shoot using ENTER button and end program using the ESCAPE button.
Now go start shooting dummies or your innocent house-mates while I am working on the computer interface to control the cannon via Bluetooth, which I will probably post here if it even gets complete. Feel free to modify the design and/or the code and share them here!