Bertie-Clay is most famous for being the inventor of the dum dum, a soft pointed bullet. At the time of their invention in 1896 Bertie-Clay was the Superintendent of the British arsenal at Dum Dum in Bengal. The Mark II Lee-Metford bullet then in use with the British Army was shown, during the Chitral Expedition of 1895, to leave a small wound with insufficient stopping power to halt a determined charge, particularly when fired at close range. The Mark II bullet had a full metal jacket and did not deform upon hitting a person, allowing it to travel straight through tissue and bone without smashing it. The "British Medical Journal" published a report on one tribesman who had been hit by six such bullets but recovered in hospital. The military authorities decided that the bullet should be altered to increase its stopping power without affecting its range. Bertie-Clay developed a bullet at Dum Dum with a metal jacket that did not run the whole length of the bullet and so exposed the softer lead tip. When this bullet hit a target the lead head would deform (or "mushroom"), spreading the force of the bullet out and causing larger wounds. In public trials held in 1896 the bullet was proved to have a greater stopping power than the Mark II. This was not the first expanding bullet, hollow-point bullets had been developed earlier particularly for use in hunting. The British Army adopted the bullet and used it against Asian and African opponents, but it was thought "too cruel" for use against Europeans. The dum dum bullet was banned by the Hague Convention of 1899 under rules that prohibited explosive bullets that had been taken over from the St. Petersburg Declaration of 1868. The delegates at the convention decided that the use of dum dum bullets in warfare was "contrary to the humanitarian spirit", despite the British delegation's protests that it was needed for colonial wars. The dum dum bullet is still in use by police forces for handguns to maximise stopping power and minimise the risk of passing through a target and hitting a bystander and in some types of game hunting.