(YouTube Link)
A fellow who calls himself "The Duckman" built an electrically-powered automatic crossbow. His objective was to have a usable crossbow now that arthritis prevents him from cocking each arrow, as he would on a conventional bow. The Duckman built the weapon with a magazine of 15 arrows and the battery allows him to fire 100 before replacement. I don't see his trigger finger move while he's firing, so it appears to be fully auto, rather than semi. More pictures and specifications at the link.
Link via Gizmodo
Over engineered? well no, he built the thing because arthritis prevents him from cocking each arrow.
And yes DUCKS!