In the field of firearms, an expanding bullet is a bullet designed to expand on impact. Such bullets are often known as Dum-dum or dumdum bullets. There are several types of dum-dum designs. Two popular designs are the hollow point (made during the manufacturing phase) and X-ing made usually by the user by making two notches perpendicular to each other on the tip of the bullet, commonly with a knife. The effect is that the bullet deforms and sometimes fragments upon impact due to the indentations. This creates a larger wound channel or channels with greater blood loss and trauma, as well as adding drag, greatly slowing the bullet, which increases the energy released into the victim and thereby greatly increases the damage due to hydrostatic shock.
In the late 19th century, the invention of Cordite ammunition permitted higher velocity than black powder, and corresponding higher hit probabilities. However to limit the amount of recoil to an acceptable level meant that higher velocity rounds needed lighter (and thus smaller diameter) bullets. Originally, dum-dum referred to a new type of ammunition produced in the early 1890s at the arsenal in Dum Dum near Calcutta, India.
Soon after the introduction of smokeless powder to firearms, full metal jacket bullets were introduced to reduce stripping by the new, smaller caliber rounds with higher velocities. 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. Since the remainder of the jacket did not cover the base of the round this could lead to the jacketing being left in the barrel and the British Indian Army produced the Mark III, Mark IV (1897) and Mark V (1899) .303 British rounds, which were of the hollow point design. These bullets expanded to a significantly larger diameter, producing larger diameter wounds than the full metal jacketed versions. Because the energy was roughly the same, none of these rounds actually produced more severe wounds than the then previous .45 Martini-Henry British service round.
The Hague Convention of 1899, Declaration III, prohibits the use in warfare of bullets which easily expand or flatten in the body, and was an expansion of the Declaration of St Petersburg in 1868, which banned exploding projectiles of less than 400 grams. These treaties limited the use of "explosive" bullets in military use, defining illegal rounds as a jacketed bullet with an exposed lead tip (and, by implication, a jacketed base). During the Convention, representatives from Imperial Germany provided evidence of severe expansion in flesh based on analysis of British hunting (not military) rounds. This provided a competitive advantage for the newly developed German Spitzer (pointed) rounds which did not have exposed lead at the tip. The United States and Britain disagreed with the German analysis, but declined to make a significant issue of it.
The competing small caliber Spitzer bullets, when at supersonic speeds, retain velocity better, giving a flatter trajectory, but have reduced terminal effect compared to expanding bullets. Spitzer bullets typically rotate or yaw after striking flesh, and then travel in a stable base forward orientation, and are referred to as "Latent Dum-dum" rounds. Theodore Roosevelt, writing about his experiences in Cuba noted that the 7 mm Mauser rounds used by the Spanish were usually significantly less lethal than the large caliber low velocity .45/70 Government rounds fired from the Allin Springfield Model 1873 "trapdoor" rifle. Unless a soldier was hit in the head, heart, or spinal cord it was very common for a soldier to take himself to the rear, and return to duty after a few days.
The First World War gave many soldiers their first exposure to high-velocity jacketed bullets of the modern type and many were unfamiliar with their sometimes dramatic effects on tissue. Wounds may be extremely large when compared to the bullet that causes them, particularly at close range, and this led some soldiers to accuse their enemies of using illegal "tampered" ammunition even when they were not.
In the 1960s, introduction of the 5.56 x 45 mm NATO cartridge caused a stir. When fired from early M-16 rifles with barrels featuring rifling cut to turn one revolution in 14 inches, these small caliber, high velocity bullets tended to tumble and fragment in soft tissue despite being of full jacketed construction. Later models of that weapon feature 1 in 12 or even faster rifling, which trade terminal ballistics for stability. In the 1980s, this controversy grew with the widespread use of the Soviet 5.45 x 39 mm in Afghanistan. The 5.45 mm bullet was developed using the experiences of the American cartridge and was specifically designed to tumble. While controversial, both bullets remain legal.
Nonetheless, some uses of expanding or exploding rounds remain legal in specific contexts. Also, many law enforcement agencies around the world use hollow pointed expanding bullets, because they are the most humane and effective bullet design for delivering maximum energy 'dump' to the subject, allowing for faster incapacitation of the subject, and thus fewer rounds fired prior to incapacitation, unlike solid bullets which tend to overpenetrate.
 Wilhelm's telegraph to Wilson
(As appearing in the 1923 Source Records of the Great War, Vol. VI, edited by Charles F. Horne.)
I feel it my duty, Mr. President, to inform you as the most prominent representative of principles of humanity, that after taking the French fortress of Longwy, my troops discovered there thousands of dumdum cartridges made by special government machinery.
The same kind of ammunition was found on killed and wounded troops and prisoners, also on the British troops. You know what terrible wounds and suffering these bullets inflict and that their use is strictly forbidden by the established rules of international law.
I therefore address a solemn protest to you against this kind of warfare, which, owing to the methods of our adversaries, has become one of the most barbarous known in history.
Not only have they employed these atrocious weapons, but the Belgian Government has openly encouraged and, since long, carefully prepared the participation of the Belgian civil population in the fighting.
The atrocities committed even by women and priests in this guerrilla warfare, also on wounded soldiers, medical staff and nurses, doctors killed, hospitals attacked by rifle fire, were such that my generals finally were compelled to take the most drastic measures in order to punish the guilty and to frighten the bloodthirsty population from continuing their work of vile murder and horror.
Some villages and even the old town of Loewen, excepting the fine hotel de ville, had to be destroyed in self-defence and for the protection of my troops. My heart bleeds when I see that such measures have become unavoidable and when I think of the numerous innocent people who lose their home and property as a consequence of the barbarous behaviour of those criminals.
- WILLIAM, EMPEROR AND KING
 See also
- Hague Convention Declaration III - On the Use of Bullets Which Expand or Flatten Easily in the Human Body July 29, 1899