M16A2 and M16A4 5.56mm Rifles: Features and Background

The Differences Between the M16-A2 and the "A4"

Marines holding guns during combat training.

 Scott Olson / Getty Images

The evolution of the Colt M-16 has been a constant change about every decade since the Vietnam War. The first M16 and Armalite AR-15 evolved into the M16A1 with modifications to the flash suppressor and attachments like the M204 grenade launcher. The M16A1 evolved to the M16A2 (aka M4) and the M16A3 and A4 which boast a list of changes that update the weapon for the varying needs of battle in the 21st century. 

Primary function: Infantry weapon
Manufacturer: Colt Manufacturing and Fabrique Nationale Manufacturing Inc.
Length: 39.63 inches (100.66 centimeters)
Weight, with 30 round magazine: 8.79 pounds (3.99 kilograms)
Bore diameter: 5.56mm (.233 inches)
Maximum effective rangeArea target: 2,624.8 feet (800 meters)
Point target: 1,804.5 feet (550 meters)
Muzzle velocity: 2,800 feet (853 meters) per second
Rate of fireCyclic: 800 rounds per minute
Sustained: 12-15 rounds per minute
Semiautomatic: 45 rounds per minute
Burst: 90 rounds per minute
Magazine capacity: 30 rounds
Unit Replacement Cost: $586

Features: The M16A2 5.56mm rifle is a lightweight, air-cooled, gas-operated, magazine-fed, shoulder- or hip-fired weapon designed for either automatic fire (3-round bursts) or semiautomatic fire (single shot) through the use of a selector lever. The weapon has a fully adjustable rear sight. The bottom of the trigger guard opens to provide access to the trigger while wearing winter mittens. The upper receiver/barrel assembly has a fully adjustable rear sight and a compensator which helps keep the muzzle down during firing. The steel bolt group and barrel extension are designed with locking lugs that lock the bolt group to the barrel extension allowing the rifle to have a lightweight aluminum receiver.

Background: The M16A2 rifle is a product improvement of the M16A1 rifle. The improvements are:

  • Heavier, stiffer barrel than the barrel of the M16A1
  • Redesigned handguard, using two identical halves, with a round contour which is sturdier and provides a better grip when holding the rifle
  • New buttstock and pistol grip made of a tougher injection moldable plastic that provides much greater resistance to breakage
  • Improved rear sight which can be easily adjusted for windage and range
  • Modified upper receiver design to deflect ejected cartridges, and preclude the possibility of the ejected cartridges hitting the face of a left-handed firer
  • Burst control device, that limits the number of rounds fired in the automatic mode to three per trigger pull, which increases accuracy while reducing ammunition expenditure
  • Muzzle compensator, designed to reduce position disclosure and improve controllability and accuracy in both burst and rapid semi-automatic fire
  • Heavier barrel with a 1 in 7 twist to fire NATO standard SS 109 type (M855) ammunition which is also fired from the M249 Squad Automatic Weapon (SAW) (This further increases the effective range and penetration of the rifle cartridge. The M16A2 will also shoot the older M193 ammunition designed for a 1 in 12 twist.)

Features of the A4 (M16A4 Rifle)

The Colt M16A4 Rifle is the fourth generation of the M16 series weapon system. Since Vietnam, the M16 has been the weapon of choice for the US and many of our allies for ground combat action. The A4, with a flat top upper receiver, a removable carrying handle, collapsible buttstock, and an integral rail mounting system for mounting of optics and other devices (lights/grenade launcher M203). 

The M16A4 Rifle in combination with the M5 Rail Adapter forms the Modular Weapon System (rifle version) which provides soldiers the flexibility to configure their weapons with those accessories required to fulfill an assigned mission. There are no differences between the internal dimensions of the M16A2 Rifle and the M16A4 Rifle.

The M4 was produced in the 1990s at the request of the US Special Forces, which is a version of the M16A2, designated M4 Carbine. The M4 has several improvements. It is basically the M16A2 rifle, fitted with a retractable buttstock, a shorter barrel, handguards, removable carrying handle for the addition of optics on the now flat top fail, and also moved the gas port back a few inches. The changes enabled the M16A3 and A4 to be quickly adjustable when needed for different mission types. 

The new updates to this weapon system enable soldiers, marines and special operations members to conduct a variety of operations from close quarters battle to a wide and open battlefield in the desert, jungle, and other environments. 

These are the changes and specifications that stayed the same. 

  • CALIBER: 5.56x45 NATO (.223 Rem.)
  • WEIGHT: 7.18 lbs (3.26 kg)
  • OVERALL LENGTH: 39.5 in. (100.33 cm)
  • BARREL LENGTH: 20 in. (50.8 cm)
  • RATE OF FIRE: 700-950 RPM