Some guns fire a single shot and are loaded manually. Others use manual repetition to load the next round. Those described here use mechanical action to automatically load and in some cases fire the next round. The subject of this document is the arrangement of mechanical action to accomplish that goal.
The term arm refers to a weapon that is hand held and can be transported by one person. Therefore a firearm is a gun intended to be used and supported by a single individual. Machine guns are usually supported by a team of at least one operator and one ammunition feeder. The machine gun is portable by one person, but the weapon system is crew-served in operation. Smaller submachine guns or fully-automatic rifles such as true assault rifles are individual weapons.
A cannon is an intermediate-sized gun, usually 20mm to 40mm in bore diameter. Cannon are included in the small arms category but are usually vehicle-borne, rather than person-carried.
The small arms category includes pistols through cannon. Guns larger than cannon are used in tanks, ships, artillery pieces and so on. They often have bore diameters 3 inches (76mm) or larger. They can be referred to as "guns", but are not considered small arms. Mortars, which provide indirect fire and operate differently, are not considered guns. Some artillery pieces can provide either direct or indirect fire. They too are not described here. Gun can be used as a generic term to refer to the entire class of weapons described here or any members of the class.
Note that the so-called "assault weapon" is a term invented by politicians seeking to ban semiautomatic versions of military-looking arms, and some "ugly" shotguns and pistols. These firearms perform no differently from "pretty" semiautomatic arms used in hunting, recreation and self-defense but are attacked due to their cosmetic similarity to and casual confusion with true military arms. Perhaps the greatest problem is that these are the exact type of arms recognized as protected for individual ownership by the U.S. Constitution. Nowhere do the words "hunting" or "sporting purpose" appear in the Constitution. The Second Amendment is contract to keep real power in the hands of the people, and not solely the government. This balances the legitimate purpose of a government to provide for a common defense on specific occasions (i.e. a war recognized by our representatives in Congress). "Assault weapon" is not a term recognized by any soldier, military officer, military scientist, small arms expert, ballistician, or properly-trained criminologist. The true assault rifle is a very specific class that is always a light rifle firing a light rifle cartridge fully-automatically. These are not readily available to the American public, though one could argue that they do have a legitimate militia purpose.
Operationally, semiautomatic firearms are derived from automatic arms. They are a special category of automatic arms where the firing cycle is interrupted so that only one round is fired each time the trigger is activated and released. Because of this relationship, the term automatic firearm can refer to either automatic or semiautomatic, or to both as a general class. However to prevent confusion, the more specific term semiautomatic should be used to describe such arms, and automatic should only be used to describe fully-automatic arms such as machine guns and submachine guns.
Fully automatic firearms are not readily available in the United States. Private ownership of fully automatic firearms is possible but the requirements are fairly onerous. In many places such requirements make them essentially impossible to own. Therefore most firearms with a military appearance one might find at a U.S. gun store are semiautomatic.
Some fully automatic guns have operating modes where they fire a single shot or a burst of 2 or 3 shots. In the former mode they operate semi-automatically, firing one round for each trigger pull. In the latter mode they operate fully automatically, firing for example 3 rounds per trigger pull.
The purpose of burst firing is to increase hit probability. That is, more rounds for a given point of aim and trigger actuation increases the likelihood of a hit, assuming the point of aim can be held relatively steadily during firing. However, the mechanism which counts shots adds mechanical, training, and operational complexity. It should also be acknowledged that with appropriate training and practice, users of regular fully automatic guns (i.e., with no burst mode) can and should be able to fire short bursts through skillful trigger manipulation.
In naval guns caliber usually refers to the length of the barrel, described as a multiple of bore diameters. A fifty caliber four inch naval gun therefore has a four inch bore diameter and a barrel 200 inches long.
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