expanding bullet is a bullet designed to expand on impact, increasing in diameter to limit penetration and/or produce a larger diameter wound. It is informally known as a Dum-dum or a dumdum bullet. The two typical designs are the hollow point bullet and the soft point bullet.
Soon after the introduction of smokeless powder to firearms, full metal jacket bullets were introduced to prevent lead fouling in the bore caused by the higher pressures and velocities when used with soft lead bullets. However, it was soon noticed that such small caliber rounds were less effective at wounding or killing an enemy than the older large caliber soft lead bullets. Within the British Indian Army, the Dum Dum arsenal produced its now infamous solution—the jacketing was removed from the nose of the bullet, creating the first soft point bullets. Since the Mark II jacket did not cover the base of the round this could potentially lead to the jacketing being left in the barrel. This potential problem resulted in the rejection of the Dum-dum design and led to independent development of the Mark III, Mark IV (1897) and Mark V (1899) .303 British rounds, which were of the hollow point design, with the jacket covering the base; while these were made in Britain, not at the Dum-Dum arsenal, the name "Dum-dum" had already become associated with expanding bullets, and continued to be used to refer to any expanding bullets. The expanding bullets expanded upon impact to a diameter significantly greater than the original .312 inch (7.92 mm) bullet diameter, producing larger diameter wounds than the full metal jacketed versions. The Mark IV was successful enough in its first use at Omdurman that British soldiers issued with the standard Mark II bullets began to remove the top of the jacket, converting the Mark II bullets into improvised Dum-dum types.
In 1898, the German government lodged a protest against the use of the Mark IV bullet, claiming the wounds produced by the Mark IV were excessive and inhumane, thus violating the laws of war. The protest, however, was based on the comparison of the wounds produced by expanding and non-expanding bullets from high velocity sporting rifles, rather than a comparison of the expanding .303 British bullets with the previous, large bore service cartridge it replaced, the .577/450 Martini-Henry. Because the energy was roughly the same, the wounds caused by the expanding bullet of the .303 were less severe than the those caused by the larger caliber, solid lead bullet used by the Martini-Henry.
The German protests were effective, however, resulting in the ban of the use of expanding bullets in warfare. The British replaced the hollow point bullets with new full metal jacket bullets, and used the remaining stocks of expanding bullets for practice.
During the Hague Convention of 1899, the British delegation attempted to justify the use of the dumdum bullet by pointing to its utility when putting down colonial unrest. Barbara Tuchman writes that, "Developed by the British to stop the rush of fanatical tribesman, the bullets were vigorously defended by Sir John Ardagh against the heated attack of all except the American military delegate, Captain Crozier, whose country was about to make use of them in the Philippines. In warfare against savages, Ardagh explained to an absorbed audience, "men penetrated through and through several times by our latest pattern of small calibre projectiles, which make a small clean holes," were nevertheless able to rush on and come to close quarters. Some means had to be found to stop them. "The civilized soldier when shot recognizes that he is wounded and knows that the sooner he is attended to the sooner he will recover. He lies down on his stretcher and is taken off the field to his ambulance, where he is dressed or bandaged. Your fanatical barbarian, similarly wounded, continues to rush on, spear or sword in hand; and before you have the time to represent to him that his conduct is in flagrant violation of the understanding relative to the proper course for the wounded man to follow - he may have cut off your head." However, the rest of the delegates at the Hague Convention 1899 did not accept this justification and voted 22-2 to prohibit the future use of the dumdum bullet.
Until relatively recently, the prohibition on the use of expanding bullets was only applicable to international armed conflicts. The International Committee of the Red Cross's customary international law study contends that customary law now prohibits their use in armed conflicts not of an international character. The adoption of an amendment to Article 8 at the Review Conference of the Rome Statute in Kampala makes the use of expanding bullets in non-international armed conflict a war crime.
Because the Hague convention applies only to the use of expanding bullets in war, the use of expanding rounds remains legal, or even required, in some circumstances. Examples of this are use of appropriately expanding bullets in hunting, where it is desirable to stop the animal quickly either to prevent loss of a game animal, or ensure a humane death of vermin, and in law enforcement or self defence, where quickly neutralizing an aggressor may be needed to prevent further loss of life, or where the bullet must remain inside the target to prevent collateral damage e.g. on an aircraft.