War remains infantry focused. Infantry needs effective rifles. Key point: Here are five of the best weapons, and how the wars of the twenty-first century changed them. 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.