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. . .
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.
The Hague Convention of 1899, Declaration III, prohibits the use in international warfare of bullets which easily expand or flatten in the body, giving as example a bullet with a jacket with incisions or one that does not fully cover the core. This prohibition was an expansion of the Declaration of St Petersburg in 1868, which banned exploding projectiles of less than 400 grams.
Because of the greater effectiveness in disabling or killing the target, 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. -- Wikipedia.
The Boffin Bullet.