10 Hallmarks of Assault Weapons — and What They Do

Guns carried by soldiers into battle have much different requirements than firearms used by civilians for hunting or target practice. An infantry soldier needs a lightweight, accurate, rapid-fire weapon with stopping power, or the ability to render a target immediately harmless. Ideally, a soldier's gun would also exhibit extreme ruggedness and reliability in battle.

Over the years, weapons manufacturers have refined their products to better meet the unique demands of infantry combat. The assault rifle stands as the perfect example. During World War II, military leaders clamored for a weapon that could give their soldiers superior firepower in assault situations. Germany cracked the nut first when it introduced the Sturmgewehr 44 (StG 44) rifle in 1943. The StG 44 came with a 30-round detachable box magazine and could operate in full automatic mode, meaning it fired bullets as long as its trigger was squeezed. It was the first true assault rifle -- and it spawned a number of similar designs, most notably the M16 in the United States and the AK-47 in Russia.

All of these weapons were made for soldiers battling other soldiers. But a funny thing happened on the way to the armistice: Hunters and gun enthusiasts saw the appeal of having a weapon with certain warlike features. As a result, gun manufacturers started making civilian versions of their military firearms. The modern AR-15 rifle, for example, evolved directly from the M16 assault rifle. It doesn't offer full automatic fire, but it otherwise looks and acts like its combat cousin.

Other guns have a similar military lineage. This heritage is a defining characteristic of assault weapons.

Many people equate assault weapon with machine gun -- a firearm capable of firing bullets as long as the operator squeezes the trigger or until the magazine empties. In reality, machine guns have been heavily regulated since the 1930s and pose less of a threat to civilians than semi-automatic weapons. A semi-automatic, or self-loading, gun is one that fires a single round every time the trigger is pulled. Confusion arises in the world of semi-automatic pistols, which also go by the name automatic pistols. In this case, however, what's automatic is how the gun loads, not how it fires.

According to the Congressional Research Service, U.S. civilians have access to 310 million firearms: 114 million handguns, 110 million rifles and 86 million shotguns [source: Krouse]. Although it's unclear how those numbers break down further, it's safe to say that semi-automatic pistols, rifles and shotguns are increasingly common today. This semi-automatic characteristic alone does not make a firearm particularly deadly, but when it's combined with high-capacity magazines (see No. 8) or certain aftermarket modifications, such as slide fire stocks (see No. 7), a mild-mannered gun can become an assault weapon.

Even without these extras, semi-automatic weapons deliver efficiency and rapid-fire results. Consider these sobering statistics: FBI studies have shown that an experienced gunman can fire a semi-automatic pistol six times in less than a second, which means he can empty a 20-round magazine in less than four seconds [source: Palmer].

Early rifles and handguns fired a single shot, a characteristic embodied by weapons like the Springfield Model 1855 rifle-musket, which saw extensive use in the American Civil War. While the Springfield was a fine weapon, the need for greater firing efficiency drove manufacturers to develop guns that could hold multiple rounds of ammunition and then chamber and fire those rounds rapidly.

All those rounds necessitated a structure to hold them, so the magazine was born. Spencer and Henry repeating rifles, or repeaters, featured magazines that ran through the butt or under the barrel of the gun respectively. These were followed by bolt-action rifles with integral box magazines, such as the Mauser Gewehr 1898 and the Springfield Model 1903. In these early weapons, the magazines held between five and 15 rounds, giving soldiers firepower they simply couldn't achieve with single-shot muskets.

The next giant leap came with the introduction of detachable magazines in semi-automatic or automatic weapons. Now a soldier could fire 15, 20, even 30 rounds of ammunition without pausing to reload. And with several magazines waiting in the wings, he could fire hundreds of rounds in a matter of minutes. Modern semi-automatic weapons used by civilians, including popular pistols such as the Glock 17, continue this tradition. Most feature removable magazines capable of holding up to 30 rounds, and a few accept high-capacity drums packed with 50 rounds. Some gun control advocates think this is too much firepower in nonmilitary situations and are seeking to ban magazines that hold more than 10 rounds.

Even if a gun comes off the line legal, it can be modified into something that pushes it into assault-weapon territory. Obviously, if a gun owner installs a grenade launcher on her AR-15 rifle, she's crossed the line. But other modifications venture into murkier water. One such modification is a replacement stock for AR-15 rifles that enables the operator to "bump fire" the gun, more safely, while it's held against the shoulder. "Bump firing" refers to a technique used by semi-automatic gun owners who want to shoot their weapon in near-automatic mode. To pull this off, someone holds the rifle at hip level, with the trigger finger held firm just in front of the trigger, and then presses the gun forward with the opposite hand. This causes the trigger to bump into his finger, which discharges the weapon and makes it recoil, which propels the rifle back against the trigger finger, allowing the gun to fire rapidly.

This technique requires a lot of practice and, in reality, poses a serious challenge to most novice gun owners. That's where a slide fire stock comes in. After a five-minute installation, this simple device provides the bump-firing experience while the weapon is held at the shoulder, where it can be operated more safely. Early versions contained springs, but the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives soon banned them. Newer versions got around the ban with a design that allows the device to slide back and forth on the rifle's buffer tube while the non-trigger hand supplies all of the spring action. Equipped with a slide fire stock, a semi-automatic rifle fires almost as rapidly as a machine gun, which is why new U.S. assault-weapon legislation is hoping to ban this type of modification in all its forms.

The stock, or buttstock, describes the part of a rifle that receives the barrel and firing mechanism and allows the weapon to be held comfortably against the shoulder. During World War I, rifle stocks had the same basic look and feel: a solid butt, a fore-end situated beneath the barrel and a grip, all fashioned from wood. The grip could be quite subtle, consisting of little more than a slight notch or pommel located just behind the trigger.

This was the way of the rifle world until 1942, when Germany introduced the Fallschirmjäger Gewehr 42, or FG 42, a weapon designed exclusively for parachute troops. The FG 42 possessed a few unique features, including a side-mounted magazine and a bipod. But one of its truly revolutionary enhancements was a sharply raked grip that extended below an in-line stock design. This marked the beginning of the pistol grip, and it gave the shooter certain ergonomic advantages, making the weapon easier to aim and improving accuracy.

The pistol grip remains a defining characteristic of assault rifles and, by extension, of assault weapons. In the ban proposed by Sen. Feinstein, any semi-automatic rifle that can accept a detachable magazine and has a pistol grip would qualify as an assault weapon. The legislation also targets thumb-hole grips, a popular workaround under the 1994 ban. A thumb-hole grip -- a stock with a hole bored through the butt just behind the trigger -- functions just like a pistol grip, delivering the same beneficial ergonomics.

In the wake of World War II, several manufacturers introduced an interesting concept -- the machine, or assault, pistol, which fell somewhere in between semi-automatic handguns and fully automatic submachine guns. Machine pistols could shoot in full automatic or burst-fire mode, but they used smaller-caliber ammunition. Even with this concession, they were notoriously difficult to control, as the recoil caused the tip of the weapon to drift.

One way to tame machine pistols is to incorporate a forward grip. The Beretta 93R, for example, comes equipped with a small kickstand-like foregrip to help improve control when it's operated in burst-fire mode. This allows the user to counteract the effects of recoil by holding the front part of the weapon with the non-trigger hand. Another trick involves covering the barrel of the weapon with a perforated tube of metal. Known as a barrel shroud, this simple addition allows a shooter to hold the weapon more securely without burning a hand on the hot barrel.

A few semi-automatic handguns still echo the designs of their fully automatic machine-pistol cousins. The Intratec TEC-DC9, or TEC-9, stands as one of the most notorious examples. Used by the shooters in the 1999 massacre at Columbine High School, the TEC-9 received its magazine in a compartment located in front of the trigger. This provided a convenient grip for anyone hoping to exert two-handed control over the weapon. Some TEC-9 models also featured barrel shrouds and shrouded barrel extensions.

Today, forward grips and barrel shrouds remain fixtures in assault-weapon legislation, despite criticism that these are cosmetic enhancements and don't contribute to the lethality of a gun.

Magazines come in two varieties: fixed and detachable. Bolt-action rifles popular in World War I, such as the British Short Magazine Lee-Enfield Mk II "Smellie," serve as good examples of the former design. The SMLE incorporated a 10-round box magazine permanently fixed to the weapon. When a soldier fired all 10 rounds, he had to feed more ammunition into the box. Clips, which allowed five to 10 rounds to be inserted as a group, made reloading easier, but it still took more time than a soldier swapping one detachable magazine for another. For these reasons, assault rifles evolved to include detachable magazines as a standard feature.

As you might expect, state and federal laws regard rifles with fixed magazines more favorably than those with the detachable variety. And yet the gun industry found ways to work around the issue. For example, a rifle equipped with a magazine lock is perfectly legal. A lock requires a tool to remove the magazine from the firearm, which makes rapid reloading much more challenging. One popular lock is known as a bullet button because it releases the magazine only when the shooter fits a bullet or cartridge into its opening. Because the bullet acts like a tool, the magazine falls into the fixed category, and the rifle remains legal. Some gun-control advocates see this as a technicality and hope to include bullet buttons as a defining characteristic of assault weapons.

Muzzle flash is a trademark of small arms fire. To understand why a weapon produces a fiery discharge, it helps to break down what happens when someone pulls the trigger of a gun. First, the trigger lever pushes the hammer backward, causing a spring to compress. As this spring uncoils, it drives the hammer forward. The firing pin on the hammer then strikes the primer, a small initiating charge incorporated into the ammunition cartridge. Upon striking, the primer generates heat to ignite the propellant, such as black powder. The propellant undergoes a process known as deflagration, which is something between burning and exploding. Deflagration produces a large amount of gas within the firing chamber, and it's this gas, under great pressure, that drives the bullet down the barrel. Once the bullet exits the barrel, the gas finally escapes, creating a flash of heat and light.

From a shooter's point of view, muzzle flash causes problems, especially at night. For example, an intense blast can give away a gunman's position. But a bigger problem occurs when the discharge blinds the shooter and diminishes his view of the battlefield. For soldiers, this can be a significant concern, which is why military rifles often come equipped with flash suppressors -- devices that attach to the muzzle and reduce the weapon's blast by directing the incandescent gases to the side.

Because flash suppressors are so common on assault rifles, gun-control advocates see this product -- or a threaded barrel designed to accept it -- as a military feature that would give civilian shooters an unwanted or unnecessary advantage. For this reason, assault-weapon legislation often includes language banning the use of flash suppressors.

One disturbing feature of assault weapons is how easily they can be transported without attracting attention. Handguns, of course, enjoy this benefit by their very nature. They're designed to be small and easy to carry and conceal. Rifles, too, have evolved in this direction. It's hard to imagine anyone sneaking around with a rifle from the late 1800s or early 1900s. The Springfield Model 1855 rifle-musket stretched 56 inches (142 centimeters) from end to end, while the Lebel M1866, used by the French infantry in World War I, measured almost 51 inches (130 centimeters) long. Both weapons would have been difficult to conceal in the back of a car or beneath a coat.

Over time, however, rifles became shorter and lighter. They also introduced new concepts, such as folding or detachable stocks, to increase their flexibility. The AK-47 assault rifle came in several designs and configurations, each offering different advantages and capabilities. In his Pulitzer Prize-winning book "The Gun," C.J. Chivers describes the flexible nature of the famous Russian firearm: "The AK-47 was small. No mule was required here. While not a precision rifle, it was accurate enough for most shots a man might be expected to take. Its ammunition was lightweight. ... Its variant with a wooden stock could be hidden beneath a blanket. The variant with a folding stock could be slung inside a coat."

Modern rifles continue the tradition. The AR-15 platform enables owners to swap out components and make modifications. One popular change involves the rifle stock and replacing the factory version with a folding or compressible stock. The folding version allows someone to reduce the length of the weapon considerably, which makes it easier to transport or carry in close quarters. The latter makes it easy to accommodate the user's body type or equipment configuration. Many states consider both stock modifications to be military features and, as a result, illegal when combined with other characteristics.

When it comes to legal language, armor-piercing bullets get all of the attention. This so-called "cop-killer" ammunition has been banned since 1986 because it can penetrate bulletproof vests. But in reality, many firearms used today have what gun enthusiasts call stopping power, a weapon's ability to render a target immediately harmless. To improve a gun's stopping power, you can do three things: increase the velocity of the bullet as it leaves the muzzle, increase the mass of the bullet being fired or design the bullet so it inflicts more trauma when it hits the flesh of the target. Rifles deliver more stopping power than pistols because they have much higher muzzle velocities. For example, the muzzle velocity of an M16 rifle is almost four times greater than a Colt M1911A1 pistol [source: Dougherty].

That doesn't mean pistols lack stopping power. In fact, most people who carry handguns for self-defense prefer larger-caliber ammunition -- .40 and .45 caliber and 9 and 10 mm -- to increase the odds that an assailant will go down and not get up [source: Kozak]. And what about gunmen bent on killing or wounding as many people as they can? They don't generally come loaded with .22-caliber target-shooting ammunition. Many of the U.S. mass shootings that have gained so much notoriety in the 21st century have included in the arsenal Glock semi-automatic pistols with larger, more powerful rounds:

It should also be noted that Jared Loughner armed his Glock with a 30-round magazine. That configuration -- semi-automatic action combined with a high-capacity magazine -- would be considered illegal under the ban proposed by Sen. Feinstein. But the weapon's stopping power is also significant, which is why we include it as a hallmark of assault weapons.