Here's What You Need to Know: These rifles delivered a lot of firepower and were used by armies across the globe.

Modern warfare has seen breathtaking advances in the last hundred years, as mortal competition between nations spawns successively deadlier weapons. Aircraft, missiles, tanks, submarines and other inventions—many of which did not exist in practical terms in 1914—have quickly earned key positions in the militaries of the world.

Yet there is still one invention that, although conceived more than five hundred years ago, still has a vital place on today’s battlefield: the infantry weapon and supporting arms. No matter how high tech the armed forces of the world have become, warfare since the end of the Second World War has consistently involved some form of infantry combat.

In his seminal work on the Korean War, This Kind of War, historian T.R. Fehrenbach wrote, “you may fly over a land forever; you may bomb it, atomize it, pulverize it and wipe it clean of life—but if you desire to defend it, protect it and keep it for civilization, you must do this on the ground, the way the Roman legions did, by putting your young men in the mud.” With that in mind, here are five of the most deadly guns of modern war.

AK-47:

The undisputed king of the modern battlefield is the Avtomat Kalashnikova model 47, or AK-47. Extremely reliable, the AK-47 is plentiful on Third World battlefields. From American rap music to Zimbabwe, the AK-47 has achieved icon status, and is one of the most recognizable symbols—of any kind—in the world. The AK series of rifles is currently carried by fighters of the Islamic State, Taliban fighters in Afghanistan, various factions in Libya and both sides in the Ukraine conflict.

The AK-47, as the story goes, was the brainchild of the late Mikhail T. Kalashnikov. A Red Army draftee, Kalashnikov showed a talent for small-arms design while convalescing from battlefield injuries, and had assembled a prototype assault rifle by 1947. (There is some circumstantial evidence, however, that the AK-47 was at least partially designed by the German designer Hugo Schmeisser, who had created the similar Stg44 in 1942.)

The AK-47 was the world’s first standard-issue assault rifle. The rifle used a new 7.62-millimeter cartridge that generated less recoil and was lighter than rounds used in traditional infantry rifles. In return, the 7.62x39 round offered more controllability when fired in full automatic and allowed the infantryman to carry more rounds into battle.

The AK-47 has endured because it is a weapon for the lowest common denominator. It requires little training to learn how to shoot, and as a result large armies or militias can be raised by simply handing out AK-47s. It is dead simple to use and requires little maintenance. Disassembly is quick and the weapon can run virtually without lubrication. All of these are important considerations when your soldiers or militiamen are often illiterate, untrained draftees.

An estimated one hundred million AK-47s of all varieties have been manufactured by countries including the Soviet Union, China, North Korea, Egypt, Yugoslavia and most of the former Warsaw Pact. As The Independent pointed out, that could be one AK for every seventy people on Earth. Even Finland and Israel, neither Soviet allies nor client states, built variants. The most recent version issued to the Russian Army is the AK-74M, chambered in the lighter 5.45-millimeter.

The M16 family of weapons:

The modern M16 rifle got its start in 1956, when inventor Eugene Stoner tested its predecessor, the AR-15, at the Infantry School at Fort Benning. The rifle would not enter U.S. service for another four years, and then with the U.S. Air Force. The U.S. Army would jump on the M16 bandwagon in 1965, with the U.S. Marines following in 1966.

The original AR-15 was a reliable, innovative rifle, but a last-minute change of gunpowder and misconceptions about the rifle’s need to be cleaned contributed to a poor reliability rate in Vietnam. Exacerbating the problem was the M16’s direct impingement self-loading system, in which gasses and carbon residue created when gunpowder is burned are cycled back into the weapon’s internal mechanism.

The most recent version, the M16A4, weighs 8.79 pounds loaded with a 30-round magazine. The rifle is effective to 550 meters, with a sustained rate of fire of 12-15 rounds per minute. The 5.56-millimeter SS109/M855 bullet, which emphasized armor-piercing capability over lethality on NATO battlefields, is being phased out in favor of the M855A1 round.

The original M16 led to the improved M16A1 by 1967, and the M16A2 by 1986. The M16A3 was a short-run rifle built for Navy SEALs, while the M16A4 has become standard issue in the U.S. Marine Corps. The M4A1 carbine, currently the standard-issue infantry weapon for the U.S. Army, is identical except for a shorter barrel, collapsible stock and the ability to be fired fully automatic.

The M16 has evolved into a reliable rifle. Modular and highly adaptable, variants have fulfilled roles from carbine to infantry rifle, squad automatic weapon and designated marksman rifle. The civilian version, again dubbed AR-15, has enjoyed explosive growth in the last ten years with the sunset of the Federal assault-weapons ban. The author owns two.

M240 Machine Gun:

The M240 Machine Gun is the current medium machine gun for the U.S. Army and U.S. Marine Corps. The M240 is in service with another sixty-eight countries and has served long enough that at least one adopter—Rhodesia—is no longer in existence.

The M240 is the American version of the FN-MAG designed in the 1950s by the Belgian arms maker Fabrique Nationale (FN). Utilizing features from both Axis and Allied infantry weapons, the MAG, as it was known, became wildly popular and standard issue with many NATO countries. In the years since introduction, the MAG has served from South Africa to the Falklands, to Afghanistan and Iraq.

The M240 can be used to engage point targets, such as individual enemy troops and light vehicles, or provide suppressive fire. The M240’s heavier 7.62-millimeter bullet gives it a maximum effective range of up to 1,800 meters.

The M240 weighs 27 pounds, and with spare barrel, tripod, and other accessories and spare parts can weigh in at up to 47 pounds. The M240 can fire 100 rounds per minute sustained fire, meaning the weapon will not overheat. It can fire up to 650 rounds per minute, but overheating is imminent.

Obviously, too many countries use the M240 to run down its use in every case, but the state of issue in the U.S. military is typical of use worldwide. In the U.S. Army, M240 machine guns are found on armored vehicles and issued at a rate of two per infantry platoon. In the U.S. Marine Corps, six guns are issued to an infantry company, allowing the company commander flexibility in their deployment.

PK Machine Gun:

The PK (Pulemyot Kalashnikova, or “Kalashnikov’s machine gun”) light machine gun was the Soviet Union’s solution to high firepower at the squad level. Like the AK-47, the PK has seen extensive use around the world—where one finds an AK, a PK is never far behind.

The PK was also invented by Mikhail Kalashnikov. Although it resembles the M240, it is in the same class as the U.S. Marine Corps’ M-27 Infantry Automatic Rifle, or the NATO Minimi/M-249 Squad Automatic Weapon.

The PK fills an important role as a Squad Automatic Weapon (SAW). Although most infantrymen throughout the world carry weapons capable of fully automatic or burst fire, trained soldiers rarely fire full auto. Full-auto fire from lightweight assault rifles is generally inaccurate and rapidly consumes ammunition.

Instead, a single gun like the PK is designated as the squad automatic weapon. The PK is equipped with a heavier barrel and frame to absorb heat and recoil from sustained fire. Accuracy, particularly mounted on a tripod, is reported to be excellent out to 800 meters.

The PK uses the same sights as the AK-47 to ease training. The PK uses a heavier hitting round than the AK-47, the 7.64x54r round originally used in the Moisin Nagant infantry rifle and Degtyaryov machine gun in World War II.

The PK has some anti-aircraft capability while the gun is mounted to a bipod or tripod, although realistically that is limited to low-flying helicopters and perhaps drones.

QBZ-95 Assault Rifle:

The QBZ-95, or “Type 95 Automatic Rifle” is the standard-issue assault rifle of China. Designed to replace the Chinese copy of the AK-47, the QBZ-95 is unlike any other Chinese rifle. The rifle is issued to all arms of the People’s Liberation Army, as well as the People’s Armed Police.

The QBZ-95 is a bullpup design, with the magazine inserted behind the trigger. Like other bullpup rifles, this shortens the overall length of the rifle. The result is a rifle with a barrel 3.5” longer than the M4 carbine, but shorter in overall length. The rifle features a built-in carrying handle, although the use of such a handle creates issues when attaching scopes and other optics.