The machine gun in 1914 could shoot 400-600 rounds a minute, operated by four to six men, had the fire power of 80 riflemen and often overheated too quickly and would jam. Water jackets and air vents helped with cooling. Even with the water jackets, some times the machine guns would over heat in just two minutes, so lots of water had to be on hand to keep the gun cool.
The first self-powered machine gun was invented by Hiram Maxim in 1885. It used the recoil of the last bullet to fire the next one. Prior to the war, the British rejected the Maxim gun, saying it was improper. Later, the British company Vickers bought Maxim and improved the Maxim gun to make it lighter and the British Army used this version instead. The Vickers Machine Gun fired .303 inch rounds, fed on cloth belts of 250 rounds. It could fire between 450 and 600 rounds a minute and was mounted on a tripod. The British did not use the machine gun very much in WWI. A few machine guns would be with each infantry battalion until late 1915, when Machine Gun Corps was formed.
The Germans quickly copied the Maxim gun to their own version, the Maschinengewehr 08. The MG 08 used belts of 250 7.9mm bullets and was usually mounted on a sledge mount. The German Armies had Machine Gun companies to assist the infantrymen.
The French saw the machine gun as too defensive and wanted to put their money towards offensive attacks instead.
A lighter machine gun was a concept that never went as planned in WWI. It was planned to be an offensive, mobile and portable machine gun. A machine gun that was light enough to be manned by a single person on the run was not ever reached. They were always too heavy. In 1914, the Lewis Light Machine Gun was developed by the British, but it was too heavy and attempts to mount it on some type of transportation was too slow. In 1918, the Germans made the Bergmann MP 18 which proved useful. The only problem was maintaining enough ammunition. Light Machine Guns were later mounted on tanks and truck In 1915, the light machine gun was mounted onto air planes and later the Vickers combined with German interrupter equipment allowed for a machine gun to shoot through the propeller of the plane.