The Chu Ko Nu is a crossbow in which the separate actions of stringing the bow, loading a bolt, and firing can be accomplished with a simple hand motion. It was the Long-Range weapon of Sun Tzu.
The Chu Ko Nu, or repeating crossbow is simply a crossbow with a wooden magazine and a lever mounted on top of it. As the lever as pulled back, the bowstring is drawn back, a bolt is loaded into the table, and the weapon is fired. The lever is then pushed forward to catch the bowstring and repeat the process. Because of the lever being mounted on top of the weapon, the repeating crossbow was braced against the user's hip.
The first crossbows in recorded history is believed to have been in 6th century BC in China. A relatively intact repeating crossbow artifact was dated to be from 4th century BC.
Although not as powerful or as accurate as a typical crossbow, it made up for these shortcomings through sheer volume of fire. An archer could fire all ten bolts in 15 seconds and fully reload the magazine in probably less time than it would take to respan an ordinary crossbow. The bolts were also tipped with poison from the wolfsbane plant.
The Qing Dynasty continued to use the Chu Ko Nu; with evidence suggesting that it was used as recently as the 1900s. However the Qing Dynasty was infamously backwards in terms of military modernization and the crossbow could not compete with modern rifles and automatic weapons.
The Chinese still had other crossbow designs that had higher pull strength, including Windlass Steel Crossbow. The Chu Ko Nu was relatively short ranged, and relied on its poison and high rate of fire to compensate.
It was tested against the Steel crossbow. While the Steel crossbow was more accurate, more powerful and, had an effective range almost twice that of the Chu Ko Nu, the Chu Ko Nu had rapid fire capabilties (fired 20 shots in span of 30 seconds) and a much fasted reload time. The Chu Ko Nu was shown to be able to penetrate the chain part of Vlad the Impaler's plated chainmail armor, although it was a glancing wound. Overall it delivered seven hits. These along with the fact Steel crossbow failed to pierce Sun Tzu's armor gave it the edge.
- Crossbows weren't actually invented during Sun Tzu's time. Crossbows came into prominence during the later half of the Warring State period in China. Sun Tzu on the other hand lived in the Spring and Autumn period hundreds of years before.