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Description
The M4 Carbine is a 14.5" barreled select fire assault rifle adopted by the United States Armed Forces. It has a distinctive 'step down' in the barrel to allow for mounting the M203 grenade launcher. The M4 has 3-round burst capability, while the M4A1 has full-auto fire capability. The very earliest M4s delivered to the US Army in 1994 had fixed carry handles and were known in Colt's catalog as the Model 777. However, by late 1994 to early 1995, the military standardized the Model 920 (M4) and Model 921 (M4A1), which feature a flattop upper receiver and a detachable carry handle; in spite of the receiver difference relative to the earlier Model 777, the Model 920 retained the same "M4" designation. Contrary to popular misconception, the carry handles on most M4s and M4A1s are detachable; since the early-2000s, almost all M4 carbines are used with the carry handle removed, and a folding rear sight and optic being attached to the receiver instead. As far as the DoD procurement program has always been concerned, the burst/auto capability is the only difference between an M4 and M4A1. (NOTE: More recently produced M4A1s - known as the Model 921HB - also have a thicker barrel profile under the handguards along with a heavier buffer to slow the rate of fire, meaning that the weapon is more controllable during fully automatic firing and is less prone to overheating.) As of 2014, most burst-fire M4s have been converted to the upgraded full-auto M4A1.

What is confusing is that many civilian commercial makers of AR-15 style rifles call their 16" Carbine "The M4" (these weapons are often referred to as "M4geries"). Though there is no rule saying they can't name their gun whatever they want, the official M4 is the version used and originally issued by the US Army and built by Colt. Colt tried to sue other gun makers to stop using the term 'M4' (specifically, Bushmaster and Heckler & Koch: the latter settled out of court and changed the name of their "HK M4" to HK416 ) however, in 2005 it was ruled that M4 refers to a type of firearm and Colt's trademark was revoked. Owing to a second spate of legal shenanigans with Colt over the US Army trying solicit new-production M4A1s competitively (which came to a head when they awarded part of the contract to Remington), M4A1s produced since February 2013 have been made by FN Herstal.

In the field, the M4/M4A1 is typically issued to standard infantry with Knight's Armament Corporation (KAC) M4 RAS railed handguards (usually holding an IR designator, a vertical foregrip, sometimes a flashlight, or one of many types of rifle scopes and red-dot or reflex optics). United States Special Operations Forces have access to additional accessories through the SOPMOD (Special Operations Peculiar Modification) program. The SOPMOD kit consists of various standardized accessories such as optics, laser sights, and suppressors, as well as a shortened M203 grenade launcher. The core of the SOPMOD is an M4A1 Carbine with a KAC railed handguard, much like the ones used by the standard infantry. The SOPMOD program, specifically Block I of the program, was initially proposed in 1992 by United States special forces, and was developed by the Naval Surface Warfare Center. Block II, which began issue in 2005, uses Daniel Defense RIS II rails (see M4/M4A1 Block II entry - below). In general usage, SOPMOD refers to M4s equipped with accessories from the SOPMOD kit, though at times the term has been used as a general name for any kind of M4 with lots of tacticool accessories.

The Mk 18 and Mk 12 (see below) had their origins in the SOPMOD program, originally intended as alternate upper receivers for the SOPMOD kit.

Description
The M4 Carbine is a 14.5" barreled select fire assault rifle adopted by the United States Armed Forces. It has a distinctive 'step down' in the barrel to allow for mounting the M203 grenade launcher. The M4 has 3-round burst capability, while the M4A1 has full-auto fire capability. The very earliest M4s delivered to the US Army in 1994 had fixed carry handles and were known in Colt's catalog as the Model 777. However, by late 1994 to early 1995, the military standardized the Model 920 (M4) and Model 921 (M4A1), which feature a flattop upper receiver and a detachable carry handle; in spite of the receiver difference relative to the earlier Model 777, the Model 920 retained the same "M4" designation. Contrary to popular misconception, the carry handles on most M4s and M4A1s are detachable; since the early-2000s, almost all M4 carbines are used with the carry handle removed, and a folding rear sight and optic being attached to the receiver instead. As far as the DoD procurement program has always been concerned, the burst/auto capability is the only difference between an M4 and M4A1. (NOTE: More recently produced M4A1s - known as the Model 921HB - also have a thicker barrel profile under the handguards along with a heavier buffer to slow the rate of fire, meaning that the weapon is more controllable during fully automatic firing and is less prone to overheating.) As of 2014, most burst-fire M4s have been converted to the upgraded full-auto M4A1.

What is confusing is that many civilian commercial makers of AR-15 style rifles call their 16" Carbine "The M4" (these weapons are often referred to as "M4geries"). Though there is no rule saying they can't name their gun whatever they want, the official M4 is the version used and originally issued by the US Army and built by Colt. Colt tried to sue other gun makers to stop using the term 'M4' (specifically, Bushmaster and Heckler & Koch: the latter settled out of court and changed the name of their "HK M4" to HK416 ) however, in 2005 it was ruled that M4 refers to a type of firearm and Colt's trademark was revoked. Owing to a second spate of legal shenanigans with Colt over the US Army trying solicit new-production M4A1s competitively (which came to a head when they awarded part of the contract to Remington), M4A1s produced since February 2013 have been made by FN Herstal.

In the field, the M4/M4A1 is typically issued to standard infantry with Knight's Armament Corporation (KAC) M4 RAS railed handguards (usually holding an IR designator, a vertical foregrip, sometimes a flashlight, or one of many types of rifle scopes and red-dot or reflex optics). United States Special Operations Forces have access to additional accessories through the SOPMOD (Special Operations Peculiar Modification) program. The SOPMOD kit consists of various standardized accessories such as optics, laser sights, and suppressors, as well as a shortened M203 grenade launcher. The core of the SOPMOD is an M4A1 Carbine with a KAC railed handguard, much like the ones used by the standard infantry. The SOPMOD program, specifically Block I of the program, was initially proposed in 1992 by United States special forces, and was developed by the Naval Surface Warfare Center. Block II, which began issue in 2005, uses Daniel Defense RIS II rails (see M4/M4A1 Block II entry - below). In general usage, SOPMOD refers to M4s equipped with accessories from the SOPMOD kit, though at times the term has been used as a general name for any kind of M4 with lots of tacticool accessories.

The Mk 18 and Mk 12 (see below) had their origins in the SOPMOD program, originally intended as alternate upper receivers for the SOPMOD kit.