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