An 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 used for hunting and by some police departments, but is prohibited for use in war. It is informally known as a Dum-dum or dumdum bullet. Two typical designs are the hollow-point bullet and the soft-point bullet.
Function and use
Expanding bullets are designed to expand on impact, sometimes as much as twice the diameter. This will slow the bullet down and more of its kinetic energy will be transferred to the target, creating a larger wound channel. For this reason expanding bullets are often used in hunting because their stopping power increase the chance of a quick kill. There are a number of designs used for hunting different game and for use in weapons with different muzzle velocities. Bullets used for medium and large game need better penetration, which means bullets designed to maintain integrity and for less expansion.  The velocity at which the bullets hit affect the expansion and penetration.
Expanding bullets are less likely to pass through the target, and if they do they will exit at a lower velocity. This reduces the risk of accidental shootings. For this reason, and to maximize the stopping effect, law enforcement organizations use expanding bullets.  Even then some penetration is needed, e.g. to penetrate a windshield or heavy clothing. 
Expanding bullets were given the name Dum-dum, or dumdum, after an early British example produced in the Dum Dum Arsenal, near Calcutta, India by Captain Neville Bertie-Clay. There were several expanding bullets produced by this arsenal for the .303 British cartridge, including soft point and hollow point designs. These were not the first expanding bullets, however; hollow point expanding bullets were commonly used for hunting thin skinned game in express rifles as early as the mid-1870s. The use of the term "Dum-dum", applied to expanding bullets other than the early .303 designs, is considered slang by some. Manufacturers have many terms to describe the particular construction of the various types of expanding bullets, though most fall into the category of soft point or hollow point designs. The expansion itself is sometimes called mushrooming.
Another early name was General Tweedie's "mushroom bullet", cited in the New York Times in 1892.
Early bullets were typically made in the form of spheres of nearly pure lead, which is a soft metal. These would often flatten upon impact with the target, causing a larger wound than the original diameter of the ball. The adoption of rifling allowed the use of longer, heavier bullets, but these were still typically constructed of soft lead and would often double in diameter upon impact. In this case expansion was a side effect of materials, and there is no evidence that the bullets were designed to expand upon impact.
The earliest examples of bullets specifically designed to expand on impact were those fired by express rifles, which were developed in the mid 19th century. Express rifles used larger powder charges and lighter bullets than typical for the time to achieve very high velocities for black powder cartridges. One method of lightening the bullets used was to provide a deep cavity in the nose of the bullet. These were the first hollow-point bullets, and in addition to developing higher velocities, they also expanded significantly upon impact. These hollow-point bullets worked well on thin-skinned game, but tended to come apart on bigger game, resulting in insufficient penetration. One solution to this was the "cruciform expanding bullet", a solid bullet with a cross-shaped incision in the tip. This split section expanded only to the depth of the incision, making it an early form of controlled expansion bullet.
In the late 19th century, the invention of Cordite and other nitrocellulose based "smokeless" propellants permitted higher velocity than black powder, along with flatter trajectories and correspondingly higher hit probabilities. However, limiting recoil to an acceptable level required higher velocity rounds to be lighter and thus smaller diameter.
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 a 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 in the battle of 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 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 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.
The Hague Convention of 1899, Declaration IV,3 prohibits the use of expanding bullets in international warfare. This is often incorrectly believed to be prohibited by the Geneva Conventions, but it significantly predates those conventions, and is in fact a continuance of the Declaration of St Petersburg in 1868, which banned exploding projectiles of less than 400 grams.
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-defense, 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.
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- India Parliament, New South Wales Parliament, Australia Parliament, Legislative Council, Parliament, Victoria (1903). Parliamentary Debates: Senate and House of Representatives. Commonwealth Govt. Printer. p. 4227. "In the dum-dum bullet the jacket ends by leaving a small piece of the core uncovered. The effect of this modification is to produce a certain extension or convexity of the point, and to give a force more pronounced than that of a bullet which is completely jacketed, at the same time, however, less effective than that of the Enfield, Snider, or Martini bullets, all of which have greater calibre."
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- Laws of War: Declaration on the Use of Bullets Which Expand or Flatten Easily in the Human Body; July 29, 1899 Document on International laws of war regarding the expanding bullet
- The .303 British Cartridge, with information on the original Dum-Dum bullet, and the subsequent MK III through MKV hollow-point bullets.
- Dum-Dum Bullet Testing, with tests of dum-dum rounds fired into jugs of water.
- Bullet Impact Comparison, results of bullets designed for different expansion and penetration