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While the definitions have evolved since the invention of gunpowder weapons, cannon are sizable crew-served weapons, which fire projectiles through a tube called a barrel. The detonation of a propellant imparts energy to propel the projectile. While some modern projectiles may include a rocket motor or glide wings to increase range, the basic source of energy are the gases generated by a low explosive fired behind the projectile.

Early cannon were smoothbore; there was no rifling to impart a stabilizing spin to the projectile. While most modern cannon are rifled, some hypervelocity weapons use fin-stabilized projectiles in smoothbore barrels.

Cannon are a subset of artillery. The term caliber, somewhat confusingly, refers to projectile weapons in two ways:

  • The inside diameter of the barrel (e.g., 5 inches/127 mm)
  • The length of the barrel in calibers (e.g., a 5"/54 caliber gun has a barrel that is 270 inches long). One of the ways to categorize cannon is by the length of the barrel:
Type Barrel length Usual firing elevation [Note 1]
Gun 30 calibers or more Low to medium [Note 2]
Howitzer 20-30 calibers [Note 3] Low to high
Mortar Less than 20 calibers [Note 4] High to very high
  • Note 1: Guns are often depressed to horizontal or near-horizontal elevation, to engage with direct fire (e.g., a tank gun), but most howitzers and some mortars can be put in a near-horizontal positions for emergencies, such as final protection of the cannon from attack
  • Note 2: Guns meant for antiaircraft use are an exception, in that they routinely assume very high elevations, for direct fire
  • Note 3: The barrel can be longer if the maximum elevation is considerably higher than is normally available for guns used in indirect fire. The term gun-howitzer is sometimes used for such weapons
  • Note 4: Some systems of nomenclature exclude all mortars from the cannon category, or only allow medium to heavy mortars, which are of a size that is usuaally vehicle-mounted or towed

Projectile and propellant systems

The first cannon fired solid cannonballs of stone or iron, which were most effective in battering walls; the advent of cannon was the beginning of the end of the stone castle. Later, various loads consisting of many small balls, variously called grapeshot or canister, were more effective against personnel targets, much like a giant shotgun. In an interesting turnabout, while solid shot were long abandoned, descendants such as armor-piercing, fin-stabilized, discarding-sabot are now used against main battle tank armor.

Early cannon, and some modern mortars, were muzzle-loaded. Most modern weapons are loaded from the opposite end, the breech.

Cannon can have fixed ammunition, where the propellant is in a cartridge case bonded to the projectile, so that they can only separate when fired. Less common in modern weapons is semifixed ammunition, where the cartridge case can be charged with variable amounts of propellant, and then inserted into the weapon and fired. A variant on semifixed ammunition is often used with light and medium mortars, where the propellant is in small containers called increments, in an open frame around the base of the shell. "Pulling increments" is the procedure for removing propellant containers from the shell, which comes with a full charge.

Finally, although rarely used in recent weapons, is separate ammunition, where the projectile is loaded, and a variable number of containers (usually silk bags) of propellant are loaded behind it. Still experimental, but reminiscent of separate ammunition, are systems that use liquid propellant, metered to the quantity required for each shot.

Autocannon fire bursts of fixed ammunition, with the complete rounds mechanically loaded from a storage device, usually called a magazine but sometimes a drum or belt.