Long arm development:
Long arms usually refer to a class of small arms which are larger than pistols, and which usually require one hand to fire and one hand to support. These terms are not strict definitions, however, and in later years a number of intermediate types of weapons have arisen.
The earliest type of long arm was probably the 火枪 or "fire lance", which the Chinese employed from the 10th century CE onwards. It featured a barrel set atop a spear, which could be filled with propellant and shrapnel and aimed at an enemy, then lit. The propellant would disperse the shrapnel load against enemies. Because of its extremely limited effective range, this was probably used as a close-range weapon, very likely to defend fortified positions as the enemy charged.
Long arms later featured a much longer barrel, and a propellant charge behind a small round projectile. These became known as muskets. Accuracy was still limited, and musket fire was only really effective when used en masse against an enemy formation, to ensure a higher probability of a hit. Reloading the musket was a time-consuming task, and infantry tactics often focused on having different soldiers open fire at different times, to allow their peers to reload while still keeping up a constant rhythm of fire to suppress their enemies.
Add a groove to the interior of the barrel (a process known as "rifling"), and the projectile would spin as it accelerated down the barrel. This helped to stabilize the projectile, and increased its effective range. The first rifles were used by hunters rather than troops, because rifling was expensive. Hunters usually hunted alert game, and increased accuracy in the first shot was vital. A single shot would alert the prey anyway, so it had to count. A rifle gave the shooter a stable firing platform, with both arms used to steady the barrel (or even resting it on terrain), and a much longer barrel than the pistol discussed above. This firearm worked well against large game, such as deer.
Against smaller game, such as fowl, the hunters discovered that firing a mass of small, spreading projectiles would be much more effective. The amount of impact needed to disable a pheasant is much lower than to disable a deer, and firing twenty pellets each time you pulled the trigger was much better than firing only one. So the shotgun evolved, firing a cup which contained a mass of small pellets. Shotgun barrels tended to be long and also quite wide (modern shotguns are more than twice the diameter of most modern pistols), and the payload would create a cloud of pellets slowly expanding as they left the barrel. Because the pellets are not aerodynamically stable, the effective range is much lower than that of a rifle or musket, but against unarmored humans or quick-moving light prey, the shotgun is an effective weapon.
Shotguns and gauge or bore: Shotgun ammunition is measured in different terminology than rifle or pistol ammunition. Each shotgun is defined by its gauge, or the diameter of its barrel. The numerical value of its "gauge" means the number of lead spheres of that diameter which it would take to measure 1 pound. This terminology arose when hunters would buy their own lead pellets from the market, and made for easy weighing and measurement. Accordingly, a larger gauge number (e.g. 28-gauge) means a smaller measurement; conversely, a smaller gauge number (e.g. 12-gauge) means a larger measurement. A 12-gauge barrel is the most common barrel, as it can fire projectiles powerful enough to take down deer ("buckshot"), whereas 28-gauge barrels are more often used against fowl.
Shotguns are almost never rifled - their interior barrel surfaces are almost always smooth (as pellets would scour and eventually ruin any rifling). A 12-gauge shotgun measures roughly 18.5mm in diameter, which allows for a versatile variety of shotgun ammunition payloads. Most common is pellets, but shotguns can fire solid slugs to take out heavy targets (such as bears). A single 12-gauge solid slug can reliably penetrate the bullet-resistant plastic shielding commonly seen in banks. Other, less common, types of payloads include sabot (fin-stabilized high-density dart, for armor piercing), flechettes (multiple solid darts), combinations of ball-and-buck (the solid slug to impact armor, and the three buck pellets following up immediately behind to cause flesh trauma), and flame rounds (which convert the shotgun into a miniature flamethrower for a few seconds).
Shotguns are also usually manual. The individual ammunition shells are so heavy that automatic shotguns suffer much more from jamming and reliability issues than other designs of firearms. In the old days, shotguns were "break action", meaning the user had to open the breech to remove the spent casings and then manually insert the fresh shells. The most common form of modern shotgun design today is the pump-action, where a tube parallel to the barrel houses a number of fresh charges, and the user pumps a mechanism which simultaneously ejects the spent shell and feeds a fresh shell into the breech. Because this relies entirely on the user's action, it is seen as highly reliable even when the shotgun has been fired many times, or is used in dirty conditions.
A number of modern shotguns are semi-automatic (or autoloading), which means the design of the gun itself uses the energy from a shot to eject the spent cartridge, and then to load in a new cartridge. However, semi-automatic shotguns require that only one, consistent, type of ammunition be used. If the user intends to use lower-energy rounds, such as beanbag rounds, the energy from each shot will be insufficient to correctly cycle the action. Also, using flame rounds in a semi-automatic shotgun would cause the cartridge to be ejected while it was still emitting a long flame. Some shotgun designers have attempted to get around this by designing a variable-action shotgun - one that can be used manually for reliability, or switched to semi-automatic for higher rate of fire. The Franchi SPAS12 is perhaps the most famous example of this - it also features a "shepherd's crook" foldable attachment that allows it to be fired one-handed. Izhmash has also released the Saiga-12 shotgun, which is essentially a larger-caliber AK-47 that fires 12-gauge shotgun shells. Although this design shares the same rugged reliability of the AK-47, the kickback limits the firearm's widespread usability. Increasing a shotgun's rate of fire simply causes too much kickback to make it worthwhile in most cases.
Fully-automatic shotguns are extremely rare and are plagued by design problems, once again relating to the relatively heavy nature of the shotgun round. Daewoo's USAS-12 attempted to upgrade the M4 carbine rifle to 12-gauge capacity (much like Izhmash did with the AK-47 to Saiga-12), but repeated jamming and reliability issues prevented the design from being taken seriously. Also, the kickback problem becomes insurmountable when firing 12-gauge munitions with autofire: merely keeping the gun pointed in the correct direction is beyond most shooters.
Rifle bullets eventually became smaller and lighter, with a much larger propellant load behind them. In WWI, rifles tended to be heavy and their bullets were designed with stability, mass, and distance in mind. Each shot had to be laboriously aimed. Firing across a battlefield at a target in a trench required accuracy, distance, and the retention of enough energy in the bullet to still kill after travelling that distance.
However, in the aftermath of WWII, the Germans and Soviets discovered that most actual exchanges of gunfire occurred at around 300m, or much less in urban areas. Thus, the design shifted to a rifle that could fire lighter bullets, and more quickly. The earliest assault rifle to see widespread use was the AK-47 (Avtomat Kalashnikov), which featured a parallel gas vent tube to the barrel. As each bullet left the barrel, the gas vent tube would redirect some of its exhaust gases backwards, to force the firing pin and bolt action back. This allowed the next bullet to be loaded into the chamber, and a heavy spring at the rear of the action would neutralize the kickback and force the bolt action forwards again, readying the weapon for its next shot.
The AK-47 was designed with loose clearances in mind, meaning the weapon could take much more abuse, neglect, and bad conditions - and still perform fairly well. However, these margins meant that high-level performance was difficult. NATO firearms tended to have tight clearances, meaning much higher performance in terms of aim, rate of fire, and bullet exit velocity - but correspondingly, they could jam, misfire, and clog with much less use than Soviet weaponry.
The British and French also discovered that bullet diameter was less important than bullet velocity, when used against human targets. The AK47's 7.62mm round was broad, and carried a significant amount of energy behind it, but it performed only middlingly well against enemy soldiers. The NATO STANAG (STANdard AGreement) rifle caliber was set at 5.56mm, a smaller round, but also a much faster exit velocity. The importance of exit velocity is that the bullet tends to shatter when suddenly decelerated in human flesh. Thus, an AK47 round might drill a neat round hole, but a STANAG round would shatter and leave a large jagged cavity in flesh. The Soviets redesigned the Kalashnikov later with the 5.45mm round, to adjust to this latest discovery.
Recently, assault rifle designs have begun to focus on miniaturization and portability. The most significant move is the adoption of "bullpup" designs: where the action and magazine are located behind the trigger, rather than traditionally in front of it. This allows the firearm greater portability, as its overall length is shorter without sacrificing barrel length (the barrel simply "originates" further back in the firearm). The mainland Chinese military has begun to move away from the familiar Kalashnikov-pattern firearm, and has adopted the QBZ-95 design, which has a bullpup configuration for ease of carrying and lighter weight. The QBZ-95 also uses a completely different caliber round from Warsaw Pact 5.45/7.62mm or NATO's 5.56mm STANAG - the Chinese have adopted a 5.8mm standard caliber, apparently finding it superior in performance to both precursors.
One main problem with bullpups is limited ambidexterity. In a full-length assault rifle, spent casings are ejected well in front of the user's face. In a bullpup assault rifle, the action and magazine are usually level with the user's face, so a left-handed user would be forced to shoot right-handed (or get hot spent casings ejected into their face or neck). Some modern bullpup firearms take this into account and eject downwards, thus removing the problem.
Note also that "bullpup" can apply to any type of firearm, not necessarily an assault rifle. Kel-Tec has even designed a bullpup pump-action shotgun, the KSG-12, which features two separate ammunition tubes, allowing the shooter to load two different types of ammunition (e.g. lethal and less-lethal).
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