Handheld crossbows with complex bronze trigger mechanisms have also been found with the Terracotta Army in the tomb of Qin Shihuang (r. 221–210 BC) that are similar to specimens from the subsequent Han Dynasty (202 BC–220 AD), while crossbowmen described in the Qin and Han Dynasty learned drill formations, some were even mounted as charioteers and cavalry units, and Han Dynasty writers attributed the success of numerous battles against the Xiongnu and Western Regions city-states to massed crossbow volleys. The bronze triggers were designed in such a way that they were able to store a large amount of energy within the bow when drawn, but was easily shot with little resistance and recoil when the trigger were pulled. The trigger nut also had a long vertical spine that could be used like a primitive rear sight for elevation adjustment, which allowed precision shooting over longer distances. The Qin/Han-era crossbow was also an early example of modular design, as the bronze trigger components were also mass-produced with relative precise tolerances so that the parts are interchangeable between different crossbows. The trigger mechanism from one crossbow can be installed into another simply by dropping into a tiller slot of the same specifications and secured with dowel pins. Some crossbow designs were also found to be fitted with bronze buttplates and trigger guard.