Below you will find a link to a digital model of my spring based shooting mechanism (https://drive.google.com/file/d/0B3K_dUGjVZSHQzVwN2YwejFUZEk/view?usp=sharing). To view it you will need LEGO Digital Designer (LDD) software, which you can download here: http://ldd.lego.com/en-us/download?ignorereferer=true. Once you opened LDD and loaded the digital model, you will be able to generate building instructions by pressing F7.
I would like to note that this is my first attempt building a digital model. There are several errors which I couldn't fix:
1. You should replace all instances of 87082 with 15100. It seems 15100 is not present in LDD.
2. The magazine is missing one part, namely 18654. The spot is easy to identify, just place it on the only exposed blue pin.
3. If you are having trouble making the bowstring or assembling the magazine, please take a look at this video: https://youtu.be/q_2lnsgdV0o.
I hope these instructions will be useful to you. Happy building!