Warfare in the post-9/11 period is primarily infantry-focused, with ground troops taking part in small-unit actions against insurgents and guerrillas. Fought on a wide variety of terrain, from arid desert regions to jungles and even cities, infantrymen have relied on their service rifles to get the mission done. Here are five of the best weapons, and how the wars of the twenty-first century changed them.

M4 Carbine:

Originally developed by Colt to fulfill a contract for the UAE, the M4 carbine was later accepted into U.S. Army and Marine Corps service. The M4 carbine is very similar to the M16A2 assault rifle, but features a shorter 14.5-inch barrel as opposed to the twenty-inch barrel of the M16. Like the M16A2, the M4 carbine fires the 5.56-millimeter round from a thirty-round magazine and has both semiautomatic and three-round-burst modes. Recently, as a result of battlefield experience with the M4, the U.S. Army decided to upgrade the weapons to the M4A1 standard. The -A1 carbines have thicker barrels for accuracy retention during sustained fire, an improved trigger, ambidextrous safety controls and the ability to fire on full automatic.

Recommended: The Colt Python: The Best Revolver Ever Made?

Recommended: Smith & Wesson 500: The Gun That Has As Much Firepower As a Rifle

Recommended: Smith & Wesson's .44 Magnum Revolver: Why You Should Fear the 'Dirty Harry' Gun

SA80A2 Rifle:

The primary service weapon of the UK’s Army, Navy and Air Force, the SA80 assault rifle was first fielded in the 1980s as a replacement for the L1A1 battle rifle. The rifle is a bullpup-configuration weapon, with the magazine and action positioned behind the trigger group. This allows for a more compact weapon system. Like the M4A1, the SA80A2 fires the 5.56-millimeter round and can accept the same magazine. Unlike the American carbine, the British one uses a short-stroke gas-piston system. In 2002, the weapons were upgraded to address a variety of shortcomings, including a modified bolt, extractor and hammer assembly, vastly increasing weapon reliability. In addition to the UK forces the SA80 has been sold to a number of Commonwealth countries, but overseas sales largely never materialized.

FAMAS:

The Fusil d’Assaut de la Manufacture d’Armes de Saint-Étienne, or FAMAS assault rifle, was adopted by France in the late 1970s. The assault rifle was a bullpup configuration, like the SA80A2, that came standard with twenty-five-round 5.56-millimeter magazines. Manufactured by GIAT, the rifle had an overall length of twenty-nine inches with a nineteen-inch barrel—nearly rifle length. The weapon weighed only 7.96 pounds. Intriguingly, unlike standard infantry assault rifles, FAMAS has radioactive tritium sights for night firing and a built-in bipod with arms that swivel up and store above the barrel. FAMAS’s blowback action gives it greater recoil than other weapons in this category. FAMAS is currently being replaced in French Army service by the Heckler and Koch 416.

HK416/M27

The Heckler and Koch 416 assault rifle is arguably a step up from the M4 carbine. Outwardly similar to the M4 in appearance, internally the rifle uses a gas-piston operating system, not the direct-impingement system used by the M4. This allows the rifle to run cooler and require less cleaning of the upper receiver, although it does reportedly make the 416 slightly front heavy. While the M4 and HK416 both share the same ammunition magazines and 5.56-millimeter round, the 416 has a longer sixteen-inch barrel, imparting a slight increase in range and velocity to the German-made weapon. The 416 is designated the M27 Infantry Automatic Rifle in U.S. Marine Corps service, and is being issued to all Marine infantrymen.

AK-47M:

The standard weapon of the Russian Ground Forces is the AK-74M. Developed in the 1970s as a replacement for the iconic AK-47, the main difference between the two weapons was the use of smaller, lighter 5.45-millimeter ammunition. The weapon, equipped with a thirty-round magazine, saw extensive use in the Soviet occupation of Afghanistan and was issued to frontline Soviet Units, particularly airborne, naval infantry and Germany-based conventional army units. The rifle has a side folding stock, 16.3-inch barrel and an overall length of thirty-seven inches. In 2015, the Russian Army adopted a number of Western-style upgrades to the AK-74M, including a skeletonized stock with adjustable cheek weld, a rail accessory mounting system similar to that on the M4 developed by Piccatinny Arsenal, foregrip and improved muzzle brake.

Kyle Mizokami is a defense and national security writer based in San Francisco who has appeared in the Diplomat, Foreign Policy, War is Boring and the Daily Beast. In 2009 he cofounded the defense and security blog Japan Security Watch. You can follow him on Twitter: @KyleMizokami.

Image: Flickr