A clever solution to the age-old problem of how to make a fully automatic crossbow. Obviously external power (in the form of a cordless drill) is needed; the device is perhaps more of a slingshot than a crossbow since it uses rubber bands rather than a steel bow. Still, one can see how a true full auto crossbow capable of penetrating the armor of those pesky invading Mongols or Turks can be devised using this system as a basis.
What would be unrealistically spiffy: a system where each crossbow bolt had its own built-in battery or capacitor. Each bolt in turn would completely discharge its power supply into the crossbow; that power would be used to operate the mechanics. This way, the full weight of the battery needed to operate the system would not need to be carried at all times; the weight of the battery would form part of the weight of the bolt itself. But this might lead to excessively expensive bolts. And of course a true replaceable box magazine would be a good addition.
Plus, one can never get tired of the true supervillain laughter of a man who takes joy and pride in the construction of fully automatic deadly weapons that manage to circumvent the laws of his native Germany.