Sarari, you are right. It is a bullet dating back to the 16th century. Todaiji Temple was involved in the civil wars between the Miyoshi and Matsnaga Clans. The army of The Miyoshi stationed in Todaiji Temple. The area around Grand South Gate became a intense battle field.
There must have been many bullets in the pillars, but now only one of them has survived. It is amazing that we can yet see and touch the 16th century bullet.