|(Part 1. AR15 - Part 2. M16 - Part 3. M4)|
Like all the variants of the AR-15 and M16, the M4 and the M4A1 can be fitted with many accessories, such as night vision devices, suppressors, laser pointers, telescopic sights, bayonets, bipods, either the M203 or M320 grenade launchers, the M26 MASS shotgun, and anything else compatible with a MIL-STD-1913 Picatinny rail. These rifles will fire the 5.56x45 NATO cartridge and the 223 Remington commercial cartridge as well.
The M4A1 carbine is a fully-automatic variant of the basic M4 carbine intended for special operations use.
The M4A1 has a "S / 1 / F" (safe / semi-automatic / fully automatic) trigger group.
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.
The M4A1 is especially favored by counter-terrorist and special forces units for close quarters combat because of the carbine's compactness and firepower. These features are also very useful in urban warfare. Although the M4 has less effective range than the longer M16, many military analysts consider engagement with a non-specialized small arm above a range of 330 yards to be unnecessary. It is effective at ranges of 160 yards or less and has a maximum effective range of about 550 to 660 yards.
In the last few years, M4A1 carbines have been refit or received straight from factory with barrels with a thicker profile under the handguard. This is for a variety of reasons such as heat dissipation, which is useful due to the complaints of high-heat production from test soldiers, which occurs during full-auto fire. These heavier barrel weapons are also fitted with a heavier buffer known as the H2.