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A machine gun is a fully automatic mounted or portable firearm, designed to fire bullets in quick succession from an ammunition belt or magazine, typically at a rate of three hundred to eighteen hundred rounds per minute. Fully automatic weapons are generally categorized as submachine guns, assault rifles, battle rifles, automatic shotguns, machine guns, or autocannons. As a class of military firearms, true machine guns are fully automatic weapons designed to be used as support weapons and generally used when attached to a mount or fired from the ground on a bipod or tripod. Light machine guns are small enough to be fired hand-held, but are more effective when fired from a prone position. The difference between machine guns and other categories of weapons is based on caliber, with autocannons using calibers larger than 20 mm,〔Marchant-Smith, C.J., & Haslam, P.R., Small Arms & Cannons, Brassey's Battlefield Weapons Systems & Technology, Volume V, Brassey's Publishers, London, 1982, p.169〕 and whether the gun fires conventional bullets, shotgun cartridges, or explosive rounds. Fully automatic guns firing shotgun cartridges are usually called automatic shotguns, and those firing large-caliber explosive rounds are generally considered either autocannons or automatic grenade launchers ("grenade machine guns"). Submachine guns are hand-held automatic weapons for personal defense or short-range combat firing pistol-caliber rounds. In contrast to submachine guns and autocannons, machine guns (like rifles) tend to have a very high ratio of barrel length to caliber (a long barrel for a small caliber); indeed, a true machine gun is essentially a fully automatic rifle, and often the primary criterion for a machine gun as opposed to a battle rifle is the presence of a quick-change barrel, heavyweight barrel, or other cooling system. Battle rifles and assault rifles may be capable of fully automatic fire, but are not designed for sustained fire. In United States gun law, ''machine gun'' is a legal term for any weapon able to fire more than one shot per trigger pull regardless of caliber, the receiver of any such weapon, any weapon convertible to such a state using normal tools, or any component or part that will modify an existing firearm such that it functions as a "machine gun" such as a drop-in auto sear.〔In United States law, a Machine Gun is defined (in part) by The National Firearms Act of 1934, as "''... any weapon which shoots ... automatically more than one shot, without manual reloading, by a single function of the trigger.''"〕 Civilian possession of such weapons is not prohibited by any Federal law and not illegal in many states, but they must be registered as Title II weapons under the National Firearms Act and have a tax stamp paid. The Hughes Amendment to the Firearm Owners Protection Act of 1986 banned new production of firearms classified as machine guns for most civilian applications, however, so only "grandfathered" weapons produced before this date are legally transferable. ==Overview of modern machine guns== Unlike semi-automatic firearms, which require one trigger pull per round fired (usually due to the sear only resetting when the trigger is released), a machine gun is designed to fire for as long as the trigger is held down. Nowadays the term is restricted to relatively heavy weapons, able to provide continuous or frequent bursts of automatic fire for as long as ammunition lasts. Machine guns are normally used against personnel, aircraft and light vehicles, or to provide suppressive fire, either directly or indirectly. They are commonly mounted on fast attack vehicles such as technicals to provide heavy mobile firepower, armored vehicles such as tanks for engaging targets too small to justify use of the primary weaponry or too fast to effectively engage with it, and on aircraft as defensive armament or for strafing ground targets, though on fighter aircraft true machine guns have mostly been supplanted by large-caliber rotary guns. Some machine guns have in practice sustained fire almost continuously for hours; other automatic weapons overheat after less than a minute of use. Because they become very hot, practically all machine guns fire from an open bolt, to permit air cooling from the breech between bursts. They also usually have either a barrel cooling system, slow-heating heavyweight barrel, or removable barrels which allow a hot barrel to be replaced. Although subdivided into "light", "medium", "heavy" or "general-purpose", even the lightest machine guns tend to be substantially larger and heavier than standard infantry arms. Medium and heavy machine guns are either mounted on a tripod or on a vehicle; when carried on foot, the machine gun and associated equipment (tripod, ammunition, spare barrels) require additional crew members. Light machine guns are designed to provide mobile fire support to a squad and are typically air-cooled weapons fitted with a box magazine or drum and a bipod; they may use full-size rifle rounds, but modern examples often use intermediate rounds. Medium machine guns use full-sized rifle rounds and are designed to be used from fixed positions mounted on a tripod. Heavy machine gun is a term originating in World War 1 to describe heavyweight medium machine guns and persisted into World War 2 with Japanese Hotchkiss M1914 clones; today, however, it is used to refer to automatic weapons with a caliber of at least .50in (12.7mm) but less than 20mm. A general-purpose machine gun is usually a lightweight medium machine gun which can either be used with a bipod and drum in the light machine gun role or a tripod and belt feed in the medium machine gun role. US Army doctrine also includes the role of "squad automatic weapon" (SAW), a weapon which is used by a single crewman and is regarded as an "automatic rifle" rather than a machine gun, though the weapon itself may actually be a machine gun in functional terms. FM 3-22.68 "Crew-Served Machine Guns", describes how the M249 can be used either as a machine gun or as an automatic rifle: "Both the M249 automatic rifle and the M249 machine gun are identical, but its employment is different. The M249 automatic rifle is operated by an automatic rifleman, but its ammunition may be carried by other Soldiers within the squad or unit. The M249 machine gun is a crew-served weapon."〔U.S. Army Training and Doctrine Command Field Manual 3-22.68 "Crew-Served Machine Guns", para. 4-207 https://rdl.train.army.mil/soldierPortal/atia/adlsc/view/public/6713-1/fm/3-22.68/chap4.htm#sec5〕 Machine guns usually have simple iron sights, though the use of optics is becoming more common. A common aiming system for direct fire is to alternate solid ("ball") rounds and tracer ammunition rounds (usually one tracer round for every four ball rounds), so shooters can see the trajectory and "walk" the fire into the target, and direct the fire of other soldiers. Many heavy machine guns, such as the Browning M2 .50 caliber machine gun, are accurate enough to engage targets at great distances. During the Vietnam War, Carlos Hathcock set the record for a long-distance shot at 7382 ft (2250 m) with a .50 caliber heavy machine gun he had equipped with a telescopic sight.〔Henderson, Charles. ''Marine Sniper'' Berkley Caliber. (2005) ISBN 0-425-10355-2.〕 This led to the introduction of .50 caliber anti-materiel sniper rifles, such as the Barrett M82. Other automatic weapons are subdivided into several categories based on the size of the bullet used, and whether the cartridge is fired from a positively locked closed bolt, or a non-positively locked open bolt. Fully automatic firearms using pistol-caliber ammunition are called machine pistols or submachine guns largely on the basis of size; those using shotgun cartridges are almost always referred to as automatic shotguns. The term personal defense weapon (PDW) is sometimes applied to weapons firing dedicated armor-piercing rounds which would otherwise be regarded as machine pistols or SMGs, but it is not particularly strongly defined and has historically been used to describe a range of weapons from ordinary SMGs to compact assault rifles. Selective fire rifles firing a full-power rifle cartridge from a closed bolt are called automatic rifles or battle rifles, while rifles that fire an intermediate cartridge are called assault rifles. Assault rifles are a compromise between the size and weight of a pistol-caliber submachinegun and a full size battle rifle, firing intermediate cartridges and allowing semi-automatic and burst or full-automatic fire options (selective fire), sometimes with both of the latter present. In certain states, like California, certain weapons that cosmetically resemble true assault rifles, but are only semi-automatic (autoloading), are categorized as "assault weapons" and possession by civilians is generally illegal. Supporters of gun rights generally consider this application of the phrase "assault weapon" to be a misnomer and this term is in fact seldom used outside of the United States for these civilian firearms. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「machine gun」の詳細全文を読む スポンサード リンク
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