The pivot catapult, known as the traction trebuchet, had existed in China since the Warring States period (as evidenced by the Mozi).[203] It was regularly used in sieges during the Han dynasty, by both besiegers and the besieged.[203] The most common projectile weapon used during the Han dynasty was the small handheld, trigger-activated crossbow (and to a lesser extent, the repeating crossbow), first invented in China during the 6th or 5th century BCE.[204] Although the nomadic Xiongnu were able to twist their waists slightly while horse-riding and shoot arrows at targets behind them, the official Chao Cuo (d. 154 BCE) deemed the Chinese crossbow superior to the Xiongnu bow.[205] http://en.wikipedia.org/wiki/Science_and_technology_of_the_Han_dynasty