AUTOMATIC: You pull the trigger and hold it, and the bullets keep flying.
SEMI-AUTOMATIC: You pull the trigger and the gun fires one bullet, then loads another bullet by itself and cocks the gun. Then you pull the trigger again. The correct term is "self-loading," but semi-automatic is fine.
ASSAULT RIFLE: A weapon that can shoot both automatic and semi-automatic. They have a selector lever to allow it to do both.
SINGLE-SHOT: Shoots one round at a time. What it means is that when you shoot the gun, you have to put another bullet in the chamber by hand or by a mechanical action and shoot again.
REPEATER: This is a dated term you won't hear often, but it refers mostly to single-shot rifles that are loaded with several bullets at once, either inside the gun itself(internal magazine) or with a clip or external magazine(see "Ammunition" below). The gun is fired, and a mechanical action is taken by the shooter to eject the shell casing and put a new round in the chamber. The next three guns below are repeaters. Note: A gun can be both single shot and a repeater at the same time.
LEVER ACTION: Some rifles(Old West Winchesters are especially distinctive) have cocking handles underneath the gun in the trigger area. After the shot, the lever is pulled down and forward, putting another bullet in the chamber and cocking the weapon.
PUMP ACTION: Used a lot on shotguns and inexpensive single-shot rifles. There's a grip at the front of the gun, under the barrel, that slides along a track. After the gun is fired, the shooter pulls the grip back toward him/herself. This ejects the shell, cocks the weapon and loads a round.
BOLT ACTION: Mechanically, this is similar to pump action. Only this time, it's done with a bolt on top of the rifle. Pull the bolt back to get the bullet in the chamber and the weapon cocked. Many sporting rifles have this kind of action, but so do much older military rifles. You may run across a lot of WWII Mausers and Mauser carbines, British Enfields, French MAS rifles, Japanese Arisakas, Russian Moisin-Nagants and American Springfields that look like this. They don't fire fast, but they're very accurate in the right hands. JFK was killed with an Italian Mannlicher-Carcano bolt-action carbine.
REVOLVER: Many standard police weapons are revolvers, although most police officers prefer the self-loading handguns. Revolvers have those familiar cylinders with six bullets therein. You pull the hammer of the revolver back until it locks. This rotates the cylinder until a bullet lines up with the barrel. You pull the trigger until the gun fires. With a double action revolver, you release the trigger and pull it again; the hammer cocks itself and fires. With single action, you have to pull the hammer back again.
ASSAULT RIFLE: The basic definition of an assault rifle is a weapon that fires both automatic and semi-automatic and uses medium-powered ammunition. They're pretty rare in the US today outside the military. Even the standard issue model of the M-16A2 in use in the US military will fire, at most, three-round bursts on one trigger pull.
The "assault" weapons used in crimes usually turn out to be something built to look like an assault rifle - AK series, M-16 and Galil are just three. SKS, an ancient Russian design, never was an assault rifle. It was merely a self-loading(semi-automatic) rifle. It is no longer made in Russia. Every "SKS" on the market today is a Chinese manufacture dumped on the US market at a low price.
Calling something an "assault rifle" is great for sales, but it can get confusing in stories. As a rule, military-looking weapons found in the public sector fire semi-automatic only, which makes them no different from a great number of ordinary hunting rifles you see in the backs of pickups.
Yes, there are illegal fully-automatic weapons around, but they're not as common as the lookalikes.
The solution? Differentiate between automatic and semi-automatic. If it has both, it's an assault rifle. If it's automatic only, it's a submachine gun. Don't even trust the police to know exactly what they have. The term "assault rifle," inaccurately applied, is becoming generic. Unless we know exactly what kind of weapon we have in a story, the safest first reference is probably something like "semi-automatic assault-style rifle." That covers most possibilities. If we can get a better definition later in the story, or in later stories, we do. If not, we're covered.
Some history: The original assault rifle was the FG42, a German paratrooper's rifle that shot both semi-automatic and fully automatic and used a medium-powered round invented for it. Pistol ammunition isn't powerful enough and rifle ammunition is so powerful it will shake the gun to pieces and be too jumpy to shoot straight. One of the most popular assault rifles in the German army was the MP43, a huge success on the Russian front. Adolf Hitler himself coined the term :assault rifle"(sturmgewehr) during testing of the MP44 late in 1944. It was the immediate forerunner of the AK-47.
AK-47: There are more misconceptions about this weapon than any other, and in a way, it's understandable. There is little difference between the various models of the same gun. But the identification became popular during the Vietnam war, when North Vietnamese army and Viet Cong troops routinely used them - or, more often, Chinese copies thereof(the Chinese copy is called the T-56).
The 47 is only the first in a long line of weapons in the Kalashnikov family, designed in 1947. It was replaced by the AK-74 in 1974 in the Soviet army, but obviously, many survive. There is also the AKM.
It's hard to tell which is which in the Kalashnikov family without getting too detailed for the reader to care. Most of them fire the same ammunition(7.62 mm), and they all have imitators. If you're not sure, you can say something is a "Kalashnikov-style rifle."
MACHINE GUNS: This is the original expression for any gun that fires at automatic rates. Now, it mostly means a portable, automatic weapon that is mounted (bipod, tripod, vehicle, boat) or laid on the ground and fired automatically only. It is not made to fire from the shoulder - that's a submachine gun. An example is the American M-60 machine gun. You'll probably always gave a better term than this to describe an automatic weapon. One of them is...
SUBMACHINE GUNS: These are personal weapons that fire pistol ammunition automatically and are carried by one shooter. They have a short range, but spew out the ammo pretty fast. The famous Tommy Gun was a submachine gun; so is the Russian PPSH-41, the original Uzi, the Sten and the Sterling. And a whole bunch of other. In general, they only in museums now. The world's military uses assault rifles.
HANDGUNS: There is much discussion about which is the correct definition of a particular handgun, and it may never be settled. In truth, there are only two types of handguns: Pistols and revolvers. The disagreement is in word usage.
Some schools say that the reader cannot distinguish a revolver from a pistol. Many of them think pistols and revolvers are one and the same. It isn't so, but that's the weight of public opinion. That school says, then, that the use of "semi-automatic handgun" is acceptable to keep the reader from getting confused. Traditionalists disagree; they say that you can only use "pistol" and "revolver" to be completely accurate. We make our own call here.
RIFLES AND SHOTGUNS: Both are shoulder-fired weapons (or at least, they're designed to be fired from the shoulder). The biggest distinction is that shotguns fire scatter shot (a bunch of little lead and steel balls of varying sizes in a plastic or paper cartridge); rifles fire a single bullet at a time. The term "rifle" comes from the spiral grooves inside the barrel that spin the bullet to stabilize it and make it travel farther and more accurately as it goes up and out of the barrel.
Note: A carbine is just a short rifle. If you call it a rifle, that's all right, because that's what it is. For what it's worth, a carbine was originally designed for horse cavalry. It fit better in a holster and was easier to carry around than the standard long rifle. Most assault rifles are carbine-sized.
Note II: We're starting to see the term "sawed-off shotgun" a lot. You have to make sure about that one. There are an awful lot of shotguns out there that do not have shoulder stocks. They look like sawed-off shotguns, but even at a quick glance, you notice there's nothing sawed-off about them. They're generally pump-action with short barrel and pistol grips - no stock. They're commonly called "riot shotguns," or "pistol grip shotguns" since the lack of a stock allows the user to swing it around in close quarters. Anyway, the 1998 US federal definition of a sawed-off shotgun is a shotgun with a barrel less than 18 inches long. Gun manufacturers get around that by making the barrels 18.5 inches long.
ROUND: This is what goes into the gun. There are three pieces: The shell, the propellant(gunpowder) and the bullet. Only the bullet goes toward the target. The shell is ejected.
CALIBERS: There are two current ways of measuring bullet diameters: millimeters and inches. Inches is an American and British development; millimeters comes from continental Europe. It's this easy: If you see a period in front of the number, as in ".22 caliber rifle," it's measured in inches. In this case, the bullet is 22/100ths of an inch thick. Therefore, the identifying measurement of the rifle is based on the ammo it takes.
How do the two systems match up? One way to remember is that .38 caliber and 9 mm are almost identical in thickness. Another: .223 caliber is the same as 5.56 millimeters. The thicker bullet doesn't necessarily have more killing power. JFK again: He was killed by a low-power, obsolete 6.5 mm round rejected in WWII by the Italian army.
Much depends on the shape and composition of the bullet's nose, the amount of gunpowder used and muzzle velocity of the gun. And, of course, where the bullet hits. This is where terms like "magnum," "wadcutter" and "soft nose" come in. Suffice it so say that they can all kill. Some just do it quicker, from a longer range and more efficiently than others. For what it's worth, the standard American military and NATO round is 5.56 mm. The older Warsaw Pact (Soviet) guns shoot 7.62 mm ammo.
CLIP: A piece of metal which is pre-loaded with ammo, then inserted into the weapon. General rule of thumb: If you can see the bullets, it's a clip. If not, it's a...
MAGAZINE: Ammo can be pre-loaded into a metal housing, then inserted into the weapon. The metal housing is the magazine. They come in different sizes and capacities.
RANGES: You'll hear the terms "low-power," "medium-power" and "high-power" referring to ammo.
-- Low-power: For pistols and submachine guns, mostly. Usually, the effective range is 100 meters or less. Sometimes, a lot less.
-- Medium-power: For assault rifles. They travel somewhere around 300-800 meters.
-- High-power: Rifles use them. They can kill from 1,000-1,200 meters. A lot of hunting rifles use them. So do military sniper rifles.