US marine firing an M4 carbine fitted with ACOG 4x scope.
Source: US Marine Corps (LCpl Mark Stroud) - © Public domain
The M4 is a late Cold War era assault rifle of US origin. It was developed as a carbine variant of the M16A2, intended for troops operating in theater, but not as frontline infantry. That role has shifted dramatically with the US Army adopting the M4 as its standard issue assault rifle. The M4 proved more portable than the M16A2, while being as effective at practical combat ranges.
The M4 is the carbine version of the M16A2, which is the NATO-caliber variant of the M16. The M4 has a shorter barrel and an adjustable length stock. The M4 retains the direct impingement gas system of the M16. The action cycles due to gas siphoned from the barrel impinging via a small tube onto a piston which is part of the bolt. The M4 is a select-fire rifle. Most variants allow for full automatic fire, with some having a similar three round burst mechanism as the M16A2. Some M4 variants are fitted with an integrated carry handle that houses the rear sights. The vast majority of M4 models are of the flattop variant. These feature a Picatinny accessory rail on top of the receiver for mounting optical or mechanical sights.
The M4 fires the 5.56x45mm NATO round from 30 round detachable magazines. Older Vietnam era 20 round magazines can also be used, as well as a wide range of newer magazine designs that are often made of polymer. Cyclic rate of fire is 700 to 950 rpm. Most M4 variants allow for full automatic fire, with some using the ratcheting three round burst mechanism. In practice semi-automatic fire is the norm, with automatic fire used only sporadically. The effective range is 300 to 400 meters, although with optics targets out to 500 meter can be engaged.
The M4 was developed at the request of the US military. It was indented to supplement the M16A2, but about decade after its initial introduction the M4 had replaced the M16A2 as standard issue assault rifle in many formations. The M4 series was also adopted by various special forces units. At present many nations around the world use the M4, or derivatives of this design. In many nations the M4 is used by special forces units, while regular units utilize a different standard issue rifle. This is partly due to the efficacy of the M4, but its image as a special forces weapon also plays a major role.
The M4 is the carbine variant of the M16A2. The M4 has a shorter barrel and retractable stock.
The Colt M4 is very similar to the older CAR-15, but is based on the NATO-caliber generation of the M16. Most variants of the M4 feature a heavier barrel than the CAR-15 and have a flattop receiver for mounting optics.
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