More interesting information from Wikipedia

What's a "dum dum bullet"?

Answer:

Dum Dum is a group of small towns to the north west of Kolkata (Calcutta) in West Bengal state in India, and the location of Netaji Subhash Chandra Bose International Airport, formerly Dum Dum Airport.

During the 19th century the area was home to a British Royal Artillery Armoury, where, in the early 1890s, Captain Bertie Clay developed a bullet with the jacket cut away at the tip to reveal its soft lead core, known as a dum dum. The British needed more powerful weapons to use against the Afridi Tribesmen in the Khyber Pass, who were apparently unconcerned by ordinary bullets. The dum dum shells were designed to expand rapidly upon impact with the human body, breaking up and inflicting a number of savage tearing wounds.

Dum dum bullets were banned by the first International Peace Conference in The Hague in 1899, along with shells that delivered asphyxiating gases, and the practice of dropping bombs from balloons "or similar new machines". The ban on dum dum bullets was the only one to survive after World War I, because they had a tendency to jam in self-loading weapons, and because armourers had by then developed fully jacketed bullets that were equally destructive in tissue.

-----------------------------

So-called "expanding bullets" were some the first "fringe" weapons banned under International Law. Their prohibition has been recently codified in Rule 77 of the ICRC's new treatise Customary International Humanitarian Law.