A machine gun uses a portion of the discharge energy from a fired cartridge to extract, reload and fire new cartridges as long as the trigger is in firing position. The operating system is generally gas or short recoil.
There are four types. The light machine gun (LMG) has a stock, weights 15-30 pounds, is air cooled, generally gas operated and is fed from a magazine or belt, in rifle caliber with a crew of two. Because of cooling issues about 100 rpm is the practical rate of fire.
The medium machine gun (MMG) can be air or water cooled, mounted on a tripod, fires a rifle caliber cartridge, weights between 25-60 pounds, has a crew of 4-5 and fires at about 500 rpm.
The general purpose machine gun (GPMG) has the same general characteristics as the LMG but has a bipod or is readily mounted on a tripod.
The heavy machine gun (HMG) has a caliber of .50 or greater, can be air or water cooled with a weight of 50 pounds or more.
Hiram Stevens Maxim was a 19th Century American genius. In what ever area he found employment: carriages, gas or electric lighting he made significant improvements. He invented an automatic sprinkler system to protect buildings from fire not very different from those in use today but while in England someone told him that if he really wanted to make money he should invent weapons, the more rapid firing the better.
In 1884, at a small factory in London, Maxim set out to use the discharge energy of the cartridge to automatically operate a weapon. The contemporary rapid fire weapons of the time were exclusively hand cranked so there were no principles to rely on. Every working idea was new, uniquely Maxims.
Maxim invented the short recoil system wherein the barrel and bolt move back together (half inch) and then a toggle unlocks the bolt for continued travel. His first prototype, using black powder, was a success. With very minor changes this system, used in the Vickers gun, saw first line service until the 1960s. He also invented the continuous belt feed system. In 1885 smokeless powder was invented, which because of its continued pressure, greatly smoothed out the operating cycle and made the blow back system for submachine guns possible.
In 1891 the British bought Maxim guns (.303 caliber, or 7.7mm). In 1893 a small contingent of Rhodesian (present day Zimbabwe) police used the guns to fight off 5,000 Matabele warriors, killing or wounding a quarter of the tribesmen. In fairly short order other European countries began to field Maxims gun. The Germans produced the Model 08 (7.92mm) at Spandau with a heavy sled mount. The Americans looked at the Maxim in 1888 but rejected it! The navy bought some 6mm Colt (potato digger) machine guns instead.
MGs traded fire for the first time in the Russo-Japanese war of 1904. Russia fielded the Maxim and Japan the Hotchkiss. Observations made during this conflict convinced European officers that there was a place for the MG not just in defense but also in the attack to provide covering fire. Some European armies began putting MGs into their infantry battalions at this time.
World War One energized the belligerents to manufacture and deploy as many MGs as possible: focusing on medium models with tripods, rifle caliber, a water jacket, belt fed and a crew of four or five.
The American Lewis gun (invented by McClean) was a gas operated LMG with a drum magazine on the top and an interesting draft air cooling jacket. They were a little heavy (26lbs) but reliable. The Brits were deploying them because they could purchase six for the cost of one Vickers. Further, it was revolutionary that a man could carry such firepower on the attack.
Although the Lewis passed US Ordnance Department tests no order came through and they even went on to obstruct further testing. Later when we entered the war, marines that had been trained with the Lewis had them taken away and were issued the French Chauchat LMG, a weapon so poorly designed and poorly made that it was excoriated by everyone who used it.
After the war Lewis sent a $1,000,000 check to the Treasury Department, stating that he did not want to make one penny of royalty from the purchase of weapons to defend his country. Gen. Crozier, Chief of Ordnance fought against this idea also (Twilight Zone music plays here in the background) but the check was finally accepted.
In 1901 Browning designed a short recoil MMG using the same principles that still operate the M2 HMG but the army had no interest. So Browning went back to civilian weapons design and an automatic rifle; the marvelous BAR.
The BAR was a light (20lbs), absolutely reliable, gas operated (Maxim invented principle) LMG with a 20 round magazine. A man could disassemble and reassemble its 70 parts in 55 seconds. Everyone who saw it demonstrated or fired it was delighted. Orders were placed by the US Army.
At the same time the army was re-introduced to the Browning MMG and decided to order this model also but they had waited too long and when war came we had 1100 obsolete MGs whereas the Germans entered the war with 12,000 new Maxims. Production of both the Brownings was just hitting their stride when the war ended.
The French Hotchkiss company after driving an unconsciousably hard bargain with an Austrian officer for his gas operating system produced a series of MMGs. They are most easily recognized by the large collar like cooling fins. The M1914 (55lbs) was fed from a sheet metal strip of cartridges and later from a belt. It was reliable and captured pieces were used by the Germans as a costal defense weapon.
They also fielded a LMG, the Benet-Mercie 1909. It was heavy (27lbs) and had a very delicate sheet metal strip feeding system. The US adopted them as the Benet-Mercie Machine Rifle. They were present in Columbus, New Mexico in 1916 when Villa attacked and contrary to some contemporary press reports they were able to fire in defense of the town.-- Peter38
The Machine-Gun: The Devils Paintbrush