This is the Maxim Gun:
In 1885, Hiram Maxim, an american inventor, demonstrated the world's first automatic portable machine-gun to the British Army.
Trials showed that the machine-gun could fire 500 rounds per minute and therefore had the firepower of about 100 rifles.
The Maxim Machine-Gun was bought by the British Army in 1889 and the following year the Austrian, German, Italian, Swiss and Russian armies also purchased Maxim's gun.