Automatic Pistols: A pistol that uses part of the energy of the firing of a cartridge to operate a mechanism that ejects the spent cartridge and loads a new cartridge from a magazine. The most widely used versions of automatic pistols all use the recoil energy of firing a cartridge, but some versions use the gas pressure inside the barrel to operate a mechanism. This usually means a (slight) loss of muzzle velocity when compared to weapons that use recoil energy. Usually a semi-automatic action, which means that the trigger has to be pulled for each time that a weapon must fire, although Fully automatic pistols are available. Automatic pistols can carry one more round than magazine capacity because one extra bullet can be carried in the gun's firing chamber.
Double Action Automatic: Semi Automatic pistol that whenever the trigger is pulled, the hammer will go back and then strike the firing pin. After the first shot, the slide will recock the hammer meaning that it does not have to be cocked for every shot. Most new automatics are of this type.
Double Action Only Automatic: Semi Automatic pistol that whenever the trigger is pulled, the hammer will go back and then strike the firing pin. After the first shot, the slide will NOT recock the hammer.
Single Action Automatic: Pulling the trigger will only drop the hammer. This means that before the first shot, the hammer must be cocked. After the first shot, the slide will recock the hammer meaning that it does not have to be cocked for every shot.
Revolvers: Revolvers are all those handguns that hold their supply of bullets in a rotating cylinder, in which chambers have been drilled. Upon firing the gun the cylinder turns to place a chamber in front of the barrel, after which the load in this chamber is fired. Upon firing the weapon again this process repeats itself. The rotating mechanism is USUALLY operated by the mechanical force that the shooter puts into pulling the trigger. However, a few weapons have (and are) being made that rotate the cylinder by using the recoil energy of the firing of the weapon
Double Action Revolver: Whenever the trigger is pulled, the hammer will go back and then strike the firing pin. You can also normally pull the hammer back manually for a more accurate shot. Most new revolvers are of this type.
Single Action Revolver: Pulling the trigger will only drop the hammer. This means that before every shot, the hammer must be cocked.
Single Shot Pistol: A pistol which holds only one round in the chamber, and has no magazine. Reloading is by hand, and can be rather time consuming. Usually reserved for very heavy cartridges, and for hunting weapons, where the need for fast reloads is not essential.
Single Shot Rifle: A Rifle which holds only one round in the chamber, and has no magazine. Reloading is by hand, and can be rather time consuming. Usually reserved for very heavy cartridges, and for hunting weapons, where the need for fast reloads is not essential.
Single Shot Repeating: Under this category fall weapons which do have a magazine, but where the shooter has to manually operate a mechanism by which the spend cartridge is ejected, and a new one is loaded from the magazine.
Submachine Guns: A weapon which fires a pistol caliber cartridge by means of a automatic mechanism. Submachine guns use a large, replaceable magazine, and some (Military and Police Models) have the ability to fire fully automatic, or in bursts. A heavier and larger weapon than a pistol, which aids in its controllability (yes, this is a vague description, as it is a vague category)
Automatic Rifles: A rifle that uses part of the energy of the firing of a cartridge to operate a mechanism that ejects the spent cartridge and loads a new cartridge from a magazine. Most automatic rifles use part of the gas pressure inside of the barrel to operate their mechanism, although recoil operated weapons of this type also exist.
Assault Rifle: This term came into being after the German Army introduced in WWII a weapon with a large replaceable box magazine, and was (relatively) crudely made, with a distinct look. It was called the MP44, or Sturmgewehr (assault rifle). Interestingly, the MP part stood for Machine Pistole, or Submachine gun..... Its current use is really a misnomer, since this nowadays refers to any Automatic rifle which has a replacable box-type magazine, and has a certain "military" look. As this has become mostly a "Politically Correct" term, without any real value, it is exceedingly unclear which automatic rifles are, and which are not "Assault Rifles". A good rifleman, or user of firearms will avoid the use of this unclear term.
Assault Weapon: As defined by US federal law, mainly any semiautomatic rifle having two or more of certain (mainly cosmetic) features common among assault rifles, such as a collapsible stock, pistol grip that extends below the main part of the frame, bayonet lug, or flash hider. It also applies to certain shotguns and handguns. The term is also used loosely to mean any rifle with a military appearance, especially ones that look like a specific assault rifle or submachine gun; this is what the media mean when they talk about "military-style" guns. The classic examples are the AR-15 (semiautomatic precursor to the M-16), and the AKM-47, AK-47S, or MAK-90 (semiautomatic versions of the AK-47). Note that assault rifles and "assault weapons" differ mainly in that the former are fully automatic and the latter are semiautomatic. This may sound like a picky technical difference, but it's huge. Functionally, assault rifles work like machine guns (i.e., can "spray bullets"), while so-called "assault weapons" are no different from many very common hunting or target rifles. The term "assault weapon" was coined by Josh Sugarmann of the (anti-gun) Violence Policy Center, precisely to engender exactly such confusion. This is not an idle accusation; he proudly boasts of fooling so many people with this ploy. Furthermore, contrary to the propaganda that they are unsuitable for sporting purposes (which is not what the Second Amendment is about anyway), they are very frequently seen at target ranges and on hunts.
Bolt Action Rifle: A rifle which has a mvable bolt, which is used to lock a cartridge into the chamber. This kind of weapon can be had with, or without, a magazine. If the weapon holds a magazine, then the action of pulling back the bolt ejects the cartridge (Spend or not!) currently in the chamber. Moving the bolt forward will strip a cartridge out of the magazine, and chamber it. In all cases the act of moving the bolt backward and forward will arm the firing pin of the weapon.
Sniper Rifle: Any rifle, be it Automatic, Bolt Action, or Single Shot which has been specially prepared to be extremely accurate at longer ranges. Usually incorporates a heavy barrel, a adjustable scope, a adjustable stock, and is often meant to fire only a few types of ammunition for best accuracy. The best such rifles can, in the hands of a skilled marksman, be used to hit persons at more than a kilometers distance.
Machine Guns: A weapon which fires rifle ammunition, with the ability to do so for long periods of fully automatic fire. Has a specially reinforced mechanism for such sustained fire, and often sports quickly inter-changeable barrels as well. The most common versions are all fed by means of a belt of linked cartridges. These weapons are often considerably heavier than a normal rifle, and are mostly used in a supportive role. Recoil at full automatic fire is great, and a Bi- or Tripod is often used to control the weapon. Rambo fires a Machine gun from the hip in his First Blood series, but this is a HIGHLY UNREALISTIC portrayal of the use of these weapons. The recoils should have spun him around, or thrown him to the ground after a few shots.
Semi Automatic: The firearm is designed so that it will fire one round for each time the trigger is pulled. This can be as rapidly or slowly as the trigger is pulled.
Fully Automatic: The firearm is designed so that when the trigger is pulled, the weapon will continue firing rounds without further trigger pulls. In the United States, fully automatic weapons are only legal to the military, law enforcement, and to a few collectors.
Caliber: Roughly speaking, the diameter of the bullet. This may be expressed in hundredths of an inch (for instance, "45 caliber" or ".45 caliber" means 0.45 inches across), or in millimeters (such as "9mm"). With handguns, generally speaking, the higher this measurement (after translating to the same units), the more powerful the cartridge used. With rifles, this rule is much less reliable, since the length of the cartridge (and thus the space available to hold gunpowder), and also the length of the bullet, can vary so much more.
(There are some quirks, however, in that some bullets are, or historically were, "heeled". This means that the bullet has a sudden small narrowing step as it enters the casing, usually resulting in the bullet being of the same diameter as the casing. Nowadays, .22 is almost the only heeled bullet in common use. .38 bullets used to be heeled, but are not any more; this is why .38 Special and .357 Magnum bullets are identical (at .357" in diameter), though the .357 Magnum cartridge is slightly longer.)
Cartridge: A complete unit of ammunition for small arms consisting of a cartridge case, primer, propellant, and projectile(s), which is inserted into the firing chamber.
Clip: A device for holding cartridges for loading
Magazine: The part of the gun where the bullets are kept for loading into the chamber. This may be an internal magazine, which does not extend outside the main body of the gun, or external, which does. It may be fixed in place, or detachable. All four combinations are possible. It is the detachable magazine, that most people mistakenly call a clip. The crucial difference between a clip and a magazine is that the magazine contains the spring that pushes cartridges up in order for them to be pushed into the chamber. into a magazine. The crucial difference between a clip and a magazine is that the magazine contains the spring that pushes cartridges up in order for them to be pushed into the gun's chamber. Therefore, the thing that most people call a clip is in fact a magazine.
Suppressor, or Silencer: A device that attaches to a firearm, for the purpose of reducing the audible sound of firing. This absolutely cannot make a firearm completely silent, since there is always some noise from the firing gases, and from the operation of the firearm (at least the striker or hammer falling, and possibly the cycling of the action). Thus, the popular term "silencer" is rather misleading, so firearm enthusiasts prefer the term "suppressor" or "sound suppressor". A gun so equipped is said to be "suppressed". Suppressors work mainly by allowing the gas to expand more slowly, and therefore with less noise, much like a car's muffler.
In the USA, these are covered by the same federal law as fully automatic guns (not illegal, but heavily regulated), so most Americans think of them as useful only for spies, assassins, and such unsavory folk. In most other countries, however, it is highly encouraged to use them for hunting, target practice, etc., so as to reduce noise pollution! (Of course, in most other countries, though the suppressor may be easy to get, the gun to put it on may be another story entirely....)
They are generally found on pistols, sometimes on rifles, on the occasional submachine gun, and once in a blue moon even on a shotgun. They are almost never found on revolvers, since the gap between the cylinder and the barrel allows gas to escape there at high velocity (and therefore high noise). There is, however, an old and fairly rare line of revolvers (Russian, late 1800s), in which the cylinder pressed forward during firing, so that the cartridge casing actually entered the barrel, forming a gas-tight seal as in the chamber of a pistol.
Maximum Effective Range: The range in which a competent and trained individual using the firearm has the ability to hit a target sixty to eighty percent of the time. This ability to hit the target is effected by the length of the barrel of the firearm, the actual cartridge fired, and quality of construction. In reality, most firearms have a true range much greater than this but the likely-hood of hitting a target is poor at greater than effective range. In the firearm lists, the effective ranges are based on personal knowledge and palladium books materials. There seems to be no good formula for the effective ranges of the various firearms.
Spray and Pray: A term often used to refer to the very poor and dangerous practice of rapidly firing many shots at a target as possible in the hope that one or more may hit the target. This practice is a danger not only to bystanders but also to the shooter as relying on luck to stop an assault can get one killed. This practice became common in law enforcement circles with the advent of large magazine capacity 9mm semiautomatic pistols. It is often referred to as "Glocking" in deference to the 17 round capacity of some Glock pistols.

The authors of The Netbook of Modern Firearms pulled some terms pulled from these pages and wish to thank the respective authors for their hard work:

Guns -- The Technical Angle by Dave Aronson.
A Glossary of Terms by Fr. Frog.

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