Artistic Forces/M4 carbine

The M4 Carbine is a lightweight, 5.56 mm caliber, air-cooled, gas-operated, direct impingement, magazine-fed, select fire carbine. It has a 14.5 in (370 mm) barrel and a telescoping stock. It is essentially a lighter and shorter variant of the M16A2 assault rifle. It is constructed of steel, aluminum and composite plastics, and is easy to break down and clean with proper training.

M4A1
The M4A1 carbine is a fully automatic variant of the basic M4 carbine intended for special operations use. The M4A1 was introduced in May 1991 and was in service in 1994. The M4A1 was the first M4 model with the removable carry handle. The M4A1 has a "S-1-F" (safe/semi-automatic/fully automatic) trigger group, while the M4 has a "S-1-3" (safe/semi-automatic/3-round burst) trigger group. The M4A1 is used by almost all U.S special operation units including, but not limited to, Marine Force Recon, Army Rangers, Army Special Forces, Navy SEALs, Air Force Pararescue and Air Force Combat Control Teams. It has a maximum effective range of 500 to 600 meters (550–660 yd). The fully automatic trigger gives a more consistent trigger pull, which leads to better accuracy. According to Mark A. Westrom, owner of ArmaLite, Inc., automatic fire is better for clearing rooms than burst fire.

The M4A1 uses a heavier barrel than the standard M4, as the regular M4 barrel, which can fire 6,000 rounds before requiring a replacement, was not sufficient for the higher consumption of ammunition by SOCOM operators. The redesigned barrel has an increased diameter in the area between the receiver and front sight.

Conversion of M4s to the M4A1 began in 2011, as part of the Product Improvement Program, which included the conversion of 300,000 M4 carbines to the M4A1. Though in service with special forces, combat in Afghanistan showed the need for providing automatic suppression fires during fire and movement for regular soldiers. The 101st Airborne Division began fielding newly-built M4A1s in 2012, and the U.S. 1st Infantry Division became the first unit to convert their M4s to M4A1-standard in May 2014. Upgrades included a heavier barrel to better dissipate heat from sustained automatic firing, which also helps the rifles use the M855A1 EPR that has higher proof pressures and puts more strain on barrels. The full-auto trigger group has a more consistent trigger pull, whereas the burst group's pull varies on where the fire control group is set, resulting in more predictable and better accuracy on semi-automatic fire. Another addition is an ambidextrous selector lever for easier use with left-handed shooters. The M4-M4A1 conversion increases weapon weight from 7.46 lb (3.38 kg) to 7.74 lb (3.51 kg), counting a back-up iron sight, forward pistol grip, empty magazine, and sling. Each carbine upgrade costs $240 per rifle, for a total cost of $120 million for half a million conversions. Three hundred conversions can be done per day to equip a brigade combat team per week, with all M4A1 conversions to be completed by 2019.

This variant of the M4 carbine is commonly used by the SDA as their service rifle that replaces the M16 rifle and its derivatives, including the M16A2, and the main armament of the Infantrymen and an upgrade for the Spec Ops Soldiers. Eventually, an under-slung grenade launcher, designated M203 was developed as an optional addition to the rifle. The 5.56mm rounds utilized by the M16 is ineffective against armored vehicles while the under-slung 40mm grenade launcher could damage lightly armored vehicles. They will eventually be replaced with the NGSW-R once an upgrade has been purchased.

Mk 18 Close Quarters Battle Receiver
The Mk 18 Close Quarters Battle Receiver is a variant of M4A1 with a 10.3-inch barrel upper receiver. It is also used by the Spec Ops Soldiers and SEALs.