What is a dum dum bullet?

It is an expanding bullet designed to grow on impact, increasing in diameter to limit penetration and/or produce a larger diameter wound. There are many expanding bullet designs though the most commonly encountered are hollow-point bullet and the soft-point bullet. Such bullets are sometimes known as ‘dum dum’ bullets. After an early British example produced in the Dum Dum arsenal, near Kolkata, India, there were several expanding bullets produced by this arsenal for the .303 British cartridge.