the thing with magazines is that they are spring loaded, and without a barrel of some sort, the bolt would just fly out when its pulled out of the mag. But anyways, this is one hell of a kickass model, better than any i've ever seen. good job mate!
Yes, there are a few and they all lack power. There are at least 8 different designs for personal units. They all lack power and range. The super old school Chinese one is really cool, but it relies on multiple people and large volleys, not power, range, and accuracy.
The only functional piece was the large siege engine type they built on Mythbusters.
The best crossbows have draws that are too far beyond the ability of compact mechanisms to set. That's why they have stirrups, because you need to stand on them to pull the cord.