Atank gun is the main armament of atank. Modern tank guns are high-velocity, large-caliberartilleries capable of firingkinetic energy penetrators,high-explosive anti-tank, andcannon-launched guided projectiles.Anti-aircraft guns can also be mounted to tanks.

As the tank's primary armament, they are almost always employed in adirect fire mode to defeat a variety of ground targets at all ranges, including dug-in infantry, lightly armored vehicles, and especially other heavilyarmored tanks. They must provide accuracy, range, penetration, and rapid fire in a package that is as compact and lightweight as possible, to allow mounting in the cramped confines of an armoredgun turret. Tank guns generally use self-contained ammunition, allowing rapid loading (or use of anautoloader). They often display a bulge in the barrel, which is abore evacuator, or a device on themuzzle, which is amuzzle brake.

The first tanks were used to break through trench defences in support ofinfantry actions particularlymachine gun positions during theFirst World War and they were fitted with machine guns orhigh explosive firing guns of modest calibre. These were naval orfield artillery pieces stripped from their carriages and mounted insponsons orcasemates on armored vehicles.

The early BritishMark I tanks of 1916 used two naval 57 mmQF 6 pounder Hotchkiss mounted either side in sponsons. These guns proved too long for use in the British tank designs as they would come into contact with obstacles and the ground on uneven terrain, and the succeedingMark IV tank of 1917 was equipped with the shortened6 pounder 6 cwt version which can be considered the first specialised tank gun.

The first German tank, theA7V, used British-made 57 mm Maxim-Nordenfelt fortification guns captured from Belgium and Russia, mounted singly at the front.

The early FrenchSchneider CA1 mounted a short 75 mm gun in a sponson on the right hand side, while theSaint-Chamond mounted a standard 75 mmfield gun in the nose. The thin armour of the tanks meant that such weapons were effective against other vehicles, though the Germans fielded few tanks anyway and the Allied tanks concentrated onanti-infantry andinfantry support activities.

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  FrenchSaint-Chamond tank of 1917, with 75 mm gun in nose
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  Replica of GermanA7V "Wotan" showing 57 mm Maxim-Nordenfelt gun in front
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  BritishMk II tank captured by German troops in April 1917, showing long 57 mm naval gun in side sponson

This thinking remained pervasive into the dawn ofWorld War II, when most tank guns were still modifications of existing artillery pieces, and were expected to primarily be used against unarmored targets.^{[citation needed]} The larger caliber, shorter range artillery mounting did not go away however. Tanks intended specifically for infantry support (theinfantry tanks), expected to take out emplacements and infantry concentrations, carried large calibre weapons to fire large high-explosive shells—though these could be quite effective against other vehicles at close ranges. In some designs – for example,M3 Lee,Churchill,Char B1 – the larger bore weapons were mounted within the tank hull while a second gun for use against tanks was fitted in a turret.

However, other strategists saw new roles for tanks in war, and wanted more specifically developed guns tailored to these missions. The ability to destroy enemy tanks was foremost on their minds. To this end, the emerginganti-tank gun designs were modified to fit tanks. These weapons fired smaller shells, but at higher velocities with higher accuracy, improving their performance against armor. Such light guns as theQF 2-pounder (40 mm) and 37 mm equipped Britishcruiser tanks and infantry tanks in the late 1930s. These weapons lacked a good high-explosive shell for attacking infantry and fortifications, but were effective against the light armor of the time.

World War II saw a leapfrog growth in all areas of military technology. Battlefield experience led to increasingly powerful weapons being adopted. Guns with calibres from 20 mm to 40 mm soon gave way to 50 mm, 75 mm, 85 mm, 88 mm, 90 mm, and 122 mm calibre. In 1939, the standard Germanpanzer had either a 20 mm or 37 mm medium-velocity weapon, but by 1945 long-barrelled 75 mm and 88 mm high-velocity guns were common. TheSoviets introduced their 122 mm in a turreted heavy tank series, theIS tanks. Shells were improved to provide better penetration with harder materials and scientific shaping. All of these meant improvements in accuracy and range, although the average tank had to grow as well to carry the ammunition, mounting, and protection for these powerful guns.

While high velocity tank guns were effective against other tanks, for the most part British tanks moved to a dual purpose 75 mm gun capable of firing a useful HE shell; later in the war adding 76 mm 17pdr gun armed tanks for better antitank capability.

Many nations devised "tank destroyers" during the war – a vehicle specifically designed for anti-tank work, and armed more heavily than a tank on the same chassis could be. They generally fell into three overlapping categories: improvised modifications of old or captured tanks to render them viable again (such as converting the machine-gun-onlyPanzer I into thePanzerjäger I), often with haphazard, poorly protected, limited-traverse weapon mounts; the American offensive and mobile reserve model, which favoured lightly-armed open-top vehicles with a rotating turret and a powerful anti-tank-capable gun while relegating true tanks to infantry support role (exemplified by theM10 tank destroyer); and the casemate gun mount model, which often allowed the resultant vehicle to be hard to hit and have a well-sloped and heavily armouredglacis plate (for instance, theSU-100). The relative superiority in armament of tank destroyers was only relative, however: for instance, theSU-85 was a casemate-type TD on theT-34 chassis that was rendered obsolete once the basic T-34 switched from the 76 mm gun to the same 85 mm cannon, producing theT-34-85.

By the end of the war the variety in tank designs had narrowed and the concept of themain battle tank emerged. The race to increase caliber slowed, with just slight increases between tank generations. In the West, guns of around 90 mm gave way to the ubiquitous 105 mmRoyal Ordnance L7, introduced in 1958. This lasted a long while, with a shift to 120 mm in the late 1970s and early 1980s (the UK changed in the late 1960s with theirChieftain tank). In the East, the 85 mm quickly yielded to the 100 mm and 115 mmU-5TS gun, with the 125 mm caliber now standard. Most of the improvements were instead made in ammunition andfire-control systems.

Withkinetic energy penetrator rounds, solid shot and armour-piercing shell gave way to armour-piercing discardingsabot (APDS) (a product of 1944), and fin-stabilized (APFSDS) rounds withtungsten ordepleted uranium penetrators. Parallel developments brought rounds based on chemical energy;high-explosive squash head (HESH), andshaped-chargehigh-explosive anti-tank (HEAT), with penetrating power independent of muzzle velocity or range.

Stadiametric range-finders were successively replaced bycoincidence andlaser rangefinders. Accuracy of modern tank guns is improved over earlier weapons by computerized fire-control systems, wind sensors,thermal sleeves, and muzzle referencing systems which compensate for barrel warping, wear and temperature. Fighting capability at night, in poor weather, and smoke was improved byinfrared,light-intensification, andthermal imaging equipment.

Technology of the guns themselves has had only a few innovations. For decades the guns were almost exclusivelyrifled, but now most new tanks havesmoothbore guns. Rifling in the barrel imparts spin on the projectile to stabilized it, improving ballistic accuracy. The best traditional antitank weapons have been kinetic energy rounds, whose penetrating power and accuracy is greatly decreased with the loss in muzzle velocity at extended range. For longer ranges high-explosive anti-tank rounds are more effective, but accuracy is limited; for extremely long rangescannon-launched guided projectiles (CLGPs) are considered more accurate.^{[citation needed]}

The use of theautoloader has been a development favoured by some nations and not others. Some countries adopted it as a means to keep the overall size of the tank down. Interest has also been shown as a means to protect the crew by separating them further from the gun and ammunition. For example, an autoloader allows the use of an unmanned turret in theT-14 Armata.

In the 1960s,smoothbore tank guns were developed by the United States, the Soviet Union, and later by the experimental American-West GermanMBT-70 joint project.

High-precision smoothbore tank gun barrels were perfected by the US Army's Weapons Laboratory at the Watervliet Arsenal based on a pair of patents by inventor Albert L. de Graffenried.^[i] More than 20,000 tank cannons were manufactured by the Watervliet Arsenal for the US Abrams M1A1 tank using de Graffenried's patented high-precision manufacturing inventions.

Based on their experience with the2A28 Grom gun/missile system of theBMP-1, the Soviets produced theT-64B main battle tank, with an auto-loaded2A46 125 mm smoothbore high-velocity tank gun, capable of firing APFSDS ammunition as well as ATGMs. Similar guns continue to be used in the latest RussianT-90, UkrainianT-84, and SerbianM-84AS MBTs.

The German companyRheinmetall developed a more conventional120 mm smoothbore tank gun which can fireLAHAT missiles, adopted for theLeopard 2, and later the U.S.M1 Abrams. The chief advantages of smoothbore designs are their greater suitability for fin stabilised ammunition and their greatly reduced barrel wear compared with rifled designs. Much of the difference in operation between smoothbore and rifled guns shows in the type of secondary ammunition that they fire, with a smoothbore gun being ideal for firing HEAT rounds (although specially designed HEAT rounds can be fired from rifled guns) and rifling being necessary to fire HESH rounds.

Most modern main battle tanks now carry a smoothbore gun. A notable exception are the tanks of the British Army which used the 120 mmRoyal Ordnance L11A5 rifled gun until the 1990s; it was then replaced it with the 120 mmL30 rifled gun which remains in service. The IndianArjun tank uses an Indian-developed 120 mm rifled gun.

• List of tank guns

• Specification and Armor Penetration Values of Soviet Tank Guns – up to the end of World War II, at the Russian Battlefield (battlefield.ru).