The term 'dum-dum bullet' is used as slang
to describe a variety of bullet
types. Unfortunately, there is no 'official' technical definition of a dum-dum, but the purpose, function and history of the bullet can be gleaned
A 'dum dum' bullet is a slang term indicating a type of bullet designed to impart greater stopping power to its target by deforming on impact. There are various means by which a bullet can be designed to do this - hollow point, soft point (also known as soft nose) or cross cut. The purpose is the same - to ensure that when the bullet strikes an object, it will flatten or 'mushroom' and hence transfer more kinetic energy to the target material rather than blasting a hole through it. Technically, the true 'dum dum' is a soft-nosed projectile - one in which the metal jacket material of the bullet stops short of the bullet's tip, leaving the soft lead exposed. These bullets are of reduced effectiveness against body armor; since the way modern ballistic armor works is to attempt to spread the force of a bullet's impact rather than allowing it to penetrate, they play to its strengths. They are enormously effective at increasing the lethality of high-velocity, smaller caliber rounds - bullets fired from long ranges, at high speeds, etc. They increase the chance of doing lethal damage with a single hit. Snipers would use them for this reason. Hunters would use them to bring down large game that might otherwise go berserk and run off (or worse, towards the hunter) wounded, if hit with a penetrating projectile. One downside to this is that the deforming lead is more likely to fragment and remain in the target, in small pieces.
History and Origins
What is most well-known about the term is also, actually, usually correct. The name 'dum-dum' is taken from the name of the Dum Dum Arsenal, run by the British Army in the Northeast of India (near Calcutta) in the nineteenth century. In 1888, the British Army adopted the Lee-Metford Mark I rifle - designed by an American, James Paris Lee, with a barrel and rifling system designed by a William E. Metford. The Mark I, I* and II of this weapon were the last black powder weapons used by the British Army, despite their modern features of a vertical box magazine and bolt action. The more powerful smokeless powder (cordite) was waiting in the wings, but would not hit the stage in wide use for a few years yet. This is of direct importance to our story.
In 1895, there was a coup d'etat in the district of Chitral, in what is now northern Pakistan. The British representative was driven from the area, which had been a British agency since 1889. This, naturally, resulted in the dispatch of an expedition from neighboring India consisting of some 16,000 troops, armed with Lee-Metfords. Although their mission was eventually successful, there were several reports that the new weapons had been not entirely satisfactory at stopping charging Chitrali tribesmen. This was due to two factors, apparently; one, the nickel-jacketed bullet used in the new gun tended to blow 'clean holes' in its targets, with little stopping power; and two, the black-powder charge in the new guns meant their muzzle velocity had not yet achieved the levels seen in more modern weapons, with a correspondingly lower kinetic energy imparted to the bullet.
In response to this, a Captain Clay at the Dum Dum Arsenal (which is where ammunition was produced for local consumption by the India station British) looked at modifying the bullets being issued for the Lee-Metford to make up for this performance gap. According to British sources (and Wikipedia), this bullet was not a hollow-point or cross-cut bullet, but a soft point - that is, the metal jacket was deleted from the very front of the projectile, leaving the lead core exposed. As a result, when the bullet struck and penetrated, the exposed lead nose would deform and flatten, transferring energy to the target more quickly. This modification was apparently only produced for local use, and not in very large numbers. The nickname 'dum-dum bullet,' however, was adopted by the local soldiery to mean a bullet modified to impart greater stopping power.
There apparently was some confusion about the meaning of the term even at the time, or perhaps more than one method was tried; a letter posted to the American Army Navy Journal in 1897 (dated November 20) indicates that the bullet is created in the following fashion: "The nickel jacket of the Lee-Metford bullet is ripped up along its length, leaving the head whole. On the impact of the new bullet, the nickel stripe and the lead spread out like a round fan and naturally cause a dreadful wound, and the person hit is immediately knocked down."
Back in England, meanwhile, the Woolwich Arsenal had been developing a hollow point bullet for the .303 Lee-Metford. This bullet was dubbed the Mark III, and adopted in 1897 - around the same time as the 'dum-dum' became available, but entirely separately, and not due to feedback from the Indian theater. It was not used very long, however, and the Mark IV and Mark V followed quickly due to design flaws, with the Mark V .303 hollow point bullet becoming standard in 1899. This may be the source of some of the confusion between hollow points and dum-dums.
The use of deforming projectiles in warfare has been a hotly-contested practice. One of the first attempts to address their use in war came almost immediately after the results of said use had been seen. At The Hague Convention of 1899, a convention was proposed and enacted in the spirit of the Declaration of St. Petersburg (1868). This new convention read, in part:
The Contracting Parties agree to abstain from the use of bullets which expand or flatten easily in the human body, such as bullets with a hard envelope which does not entirely cover the core, or is pierced with incisions.
The present Declaration is only binding for the Contracting Powers in the case of a war between two or more of them.
It shall cease to be binding from the time when, in a war between the Contracting Parties, one of the belligerents is joined by a non-Contracting Power.
Note that the United States was not a signatory to this document. However, the U.S. did sign on to The Hague Convention of 1907. Article 23 of that document states that "In addition to the prohibitions provided by special Conventions, it is especially forbidden -...To employ arms, projectiles, or material calculated to cause unnecessary suffering." This depends on whether the use of the dum-dum is 'calculated to cause unnecessary suffering.' Note that originally, it had nothing to do with suffering and everything to do with effectiveness; it could be argued that the much higher lethality of the dum-dum might involve less suffering than the wounds caused by FMJ rounds. However, on such fine points are great drunken arguments and international incidents made. Kaiser Wilhelm wrote President Woodrow Wilson a blistering note regarding the reported use of dum-dum bullets by the Belgian forces during the German invasion of their country in 1914, which the Belgians denied. So the degree to which this prohibition exists beyond its use as a diplomatic lever is uncertain.
It should be noted that, for the most part, modern militaries do, in fact, use fully jacketed bullets. This is not true in all cases, however. For example, in 1985, the U.S. Judge Advocate General issued an opinion which specifically stated that the use of a particular type of bullet (Sierra #2200 'Match King') by snipers, which had an open tip (soft nose), was not in contravention to the Laws of War because the open tip was used to increase accuracy rather than increase suffering or bullet effects, and that comparison of the effects of these bullets to jacketed bullets showed a negligible variation. In addition, since the U.S. has generally considered itself bound by the 1899 document despite not being a formal signatory, it is probably with some sense of delicious irony that an oft-mentioned (but not seen by this author) 1990 document (also purportedly from the JAG, and the same authors therein of the 1985 memo) concluded that in counterterrorist operations, open-tipped bullets were A-OK since the opposing forces were definitely of a 'non-contracting Power.'
The Hague Conventions - http://www.yale.edu/lawweb/avalon/lawofwar/lawwar.htm
A.E. Hartink, Encyclopedia of Rifles and Carbines. Rebo Productions: The Netherlands, 1997.
On War: Armed Conflict Event Data - http://www.onwar.com/aced/data/charlie/chitral1895.htm
Army Navy Journal - http://www.armedforcesjournal.com/AFJI/history/Mags/2002/march02/march.html
The Gun Zone - http://www.thegunzone.com/opentip-ammo.html
FirstWorldWar.Com - http://www.firstworldwar.com/source/kaiserdumdumbullets.htm
Military Rifles - http://www.militaryrifles.com/Britain/Metford.htm