That's a feeder on top and the lever near the end is the repeating action. The repeating crossbow was the first semi-automatic weapon ever invented. Trained soldiers could let fly 10 bolts in 15 seconds before the magazine needed to be reloaded. Due to the nature of the action, it needed to be shot from the hip in order to fire that quickly. Oh, and the bolts were often dipped in a fast acting poison so that just a scratch could be fatal. Fighting in small teams reduced the risk of friendly fire.
Siege warfare was all the rage when this baby hit the scene. It proved to be an essential defensive weapon as one could keep backing away from the enemy while shooting wildly into the advancing horde.
Seriously, just look at that s**t. It was a damned crossbow assault rifle, and it existed so long ago Jesus could have owned one. An old one.
"Jesus Christ! Cover me!"