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An elephant gun is a large caliber gun, rifled or otherwise, so named because they were originally developed for use by big-game hunters for elephants and other large dangerous game. They used black powder cartridges at first but later started using smokeless powder. They were sometimes used in wars.
Early use 
As Europeans made inroads into Africa in the early 19th century, guns were developed to handle the very large game encountered. This was for self-protection, food gathering, and later, more commonly, sport. The first guns were the simple muzzle-loading shotgun designs already used for birds and loaded with solid balls of lead for use on large game. Due to their ineffectiveness on the largest game (up to 35 shots being recorded by some writers for a single elephant), they soon developed into larger caliber black powder smoothbores. The caliber was still measured in bore or gauge — 10, 8, 6, 4 bore, or even 2 bore — or the guns were named by projectile weight in ounces. The projectiles were lead round balls or short conical slugs, sometimes hardened with antimony.
These very large and very heavy firearms were the first to be known as the elephant guns of the black powder era (1850–1890), though their use also included all thick-skinned dangerous game such as rhinoceros, hippopotamus and cape buffalo. Due to the velocity limitations of black powder and lead—usually around 1,500 feet per second (460 m/s)—the only way to increase penetration was to make a larger gun. The largest bore guns in common use (and bore rifles with the advent of breech loading and rifling in the late 19th century) included the 4 bore- using a 2,000-grain (130 g) slug at up to 1,400 ft/s (430 m/s). Despite their enormous power, the short low velocity slugs still suffered the penetration issues which plagued guns of this era, particularly for the toughest shot of all: defeating the bone mass for a frontal brain shot on an elephant. Thus, dangerous game hunting in the 19th century was as much a test of the gun-bearer's ability to relay guns to the hunter, and his skill on horseback in the earlier days to evade charges long enough to reload.
It was not until the parallel developments of jacketed projectiles, closely followed by smokeless powders in the late 19th century, that dangerous game could be taken with near 100% certainty.
Nitro Express rifles 
The Nitro Express line (c.1895), so named because the composition of the early smokeless powders such as Poudre B, ballistite and cordite, were the first of the new order of elephant guns. With smaller metal jacketed projectiles ranging from .400 to .620 and velocities around 2,000 ft/s (610 m/s) they possessed vastly improved trajectory and penetration over their black powder forebears. Within a few short years the mighty bore guns of the previous era largely disappeared from the gamefields. The safari heyday of the early 20th century 'Nitro era' records much literature on such calibres as the .577 Nitro Express, .375 H&H Magnum, .416 Rigby, .404 Jeffery, .505 Gibbs, .450 Nitro Express, .470 Nitro Express and many others. These rifles came out in single shot, bolt action, and double rifle configuration and continued to be used up until ivory hunting died off in the mid 20th century. Thereafter, they largely switched roles to tools for game wardens and as back-up firearms for professional hunters guiding international hunters.
The American gun market produced several famous dangerous game cartridges around this time, such as the .458 Winchester Magnum, .378 Weatherby Magnum and .460 Weatherby Magnum and many of these were 'wildcatted' (to modify an existing case and rifle to fire a different caliber bullet). The rest of the old Nitro express calibers were to fade almost to obscurity until a recent resurgence in safari hunting came about in the 1970s and 1980s. This prompted a new boom in elephant gun development and calibers such as the .416 Weatherby Magnum and .416 Remington Magnum arrived in factory offerings. The late 1980s and 1990s produced the .700 Nitro Express and the new brass manufacturers allowed even more powerful elephant guns such as the .585 Nyati by Ross Seyfried, .577 Tyrannosaur by Colonel Art Alphin and .585 Gehringer by Karl Gehringer to be made by wildcatters. The .600 Overkill made by Rob Garnick represents at this moment the greatest power available from a standard hunting action. Other wildcats based on the heavy machine gun .50 BMG and similar anti-materiel rounds have been devised which are much more powerful, though they are not generally considered useful hunting arms being that their weight usually exceeds 25 lb (11 kg).
Whether double rifle, single shot, or bolt action the concept of the elephant gun is the same: to provide enough stopping power to prevent harm to the hunter in the case of charging game. The necessities for the gun are not only extreme power, since in that case the 50BMG or 20 mm cannons would be the order of the day, but that it can be carried for long periods, shot from any position or angle, and be well balanced enough to track on rapidly moving animals. In essence it is no more than a very large hunting rifle with the same capability of use as any hunting rifle.
Use in war 
During World War I, both the British and Germans deployed elephant guns obtained from their African colonies in an attempt to break the stalemate in the trenches. The British used elephant guns as a means of countering the German tactic of having their snipers advance towards Allied lines under the cover of a large, 6-10 millimeter (0.24-0.4 inch) thick steel plate. Though normal small arms were ineffective against the plate, the elephant guns of the era had enough force to punch through it. Likewise, the Germans deployed a specialized, mass-produced anti-tank rifle, the Mauser 1918 TuF Gewehr, to knock out lightly armored British tanks.
During the African campaigns of World War II in 1941, the Italians in East Africa faced the British. The Italian commander, the Duke of Aosta, gave his personal collection of elephant guns to his Italian soldiers to aid in armour penetration of British armoured cars, as Italian AT guns were in short supply.
The Finnish 20 mm anti-tank gun Lahti L-39 gained the nickname Norsupyssy (Elephant Gun) during the Continuation War because of its stopping power. It is not a true elephant gun, though, since it was not designed for pachyderm hunting but as a purely military weapon.
In the 21st century, more elephants are shot by impoverished poachers than by wealthy trophy hunters. Due to quirks of the arms trade and various civil wars, Kalashnikov assault rifles have become relatively cheap in Africa and possibly in Southeast Asia, thus making exotic large-caliber rifles largely historical curiosities.