The FN M16A4 would officially replace the M16A2 as the USMC standard service rifle in 1997. With a Picatinny upper receiver and Knights Armament quad rails, the M16A4 would provide the Marine Corps with the ability to add infrared aiming devices, flashlights and optics to their service rifles. While implementation of the “A4” would not begin in large quantities until after the beginning of the Global War on Terror (GWOT), the design would become synonymous with the Marines during this timeframe.

M16A4 and ACOG during OIF I with 1st Battalion, 5th Marines (USMC).

The new “flat top” Picatinny railed upper receiver and Knights Armament M5 Rail Adapter System (RAS) would be the only changes from the M16A2 platform.

Marines with 1/6 during the early years of Operation Enduring Freedom in Afghanistan. Some have the removable carry handle rear sight on their M16A4s, while others have a Trijicon Advanced Combat Optical Gunsight (ACOG) mounted to the top of their carry handles. The rifles with larger scopes are M16A4 SAM-R Squad Advanced Marksman Rifles (USMC).

By Operation Iraqi Freedom II in 2004, the M16A4 was widely issued amongst Marine infantry units. The Trijicon ACOG TA31F was the primary optic, and the AN/PEQ-2 infrared aiming device was a standard attachment. From there, variances were allowed based on the individual Marine’s preference. Some Marines would use “broomstick” attachments on their M5 RAS rails to better hold their rifles, a very useful attachment for the urban combat the Marine Corps would experience in Iraq. The urban fighting would also make Surefire rail mounted flashlights a necessity, and several generations of Surefires would be used on USMC M16A4s. In general, the rifle would perform well in combat.

Marines with M16A4s during OIF II. The Second Battle of Fallujah would be the most intense urban warfare American troops would face in the Global War on Terror. Some Marine units would clear houses with bayonets fixed to their M16A4s due to this fact (USMC).

The ACOG would revolutionize service rifles in general, and provide the USMC with a distinct advantage in combat. The first version the Marine Corps would field would be Trijicon’s ACOG TA31F, which is the version seen in Iraq and Afghanistan in 2003-2005. In 2006 the Marine Corps would begin using the ACOG RCO (Rifle Combat Optic). The RCO was designed specifically for the Marine Corps and came in 2 versions: one for the M16A4 and another for the M4A1. The difference between the two designs can be found in the Bullet Drop Compensator (BDC), with the M4 RCO’s BDC calibrated for a 62 grain bullet and a 14.5″ barrel and the A4 RCO for a 20″ barrel. Other shared improvements were wired scope caps, additional ranging stadia and a tenebraex kill-flash to eliminate glare.

Marines in Iraq, 2005-2008 with M16A4s. In addition to the newer ACOG RCOs, the Grip Pod, a vertical foregrip like the Knight’s Armament “broomstick” had an integral bi-pod that could be deployed. USMC M16A4s with Grip Pods were commonly seen from 2006-2009 (USMC).

Marines in Afghanistan and on Marine Expeditionary Unit (MEU) deployments, 2008-2011. The M16A4 seen during this era of Operation Enduring Freedom reflects the trend towards lighter-weight Marine infantry in general (USMC).

While the USMC had been active in Afghanistan since 2001, the war in Iraq would dominate deployments until 2008. In 2009 with the situation in Afghanistan deteriorating, Marines would be deployed to the nation for some of the most intense combat of the GWOT. The M16A4 carried into Afghanistan in this era would be mostly the same as the one carried in Iraq. The PEQ-2 infrared aiming device would be upgraded to the PEQ-15 and shortly thereafter the PEQ-16. Back-Up Iron Sights were most often the USMC marked Knights Armament type. The rest of the rifle would stay the same, but the M16A4s eventual replacement, the Heckler and Koch M27 Infantry Automatic Rifle (IAR) would began to be fielded by infantry units in 2011.

The AN/PEQ-16 and Knights Armament USMC Back-Up Iron Sights would be the main upgrades to USMC M16A4s in the late stages of frontline service use.

The M27 IAR would be issued to infantry units deploying to Afghanistan in 2011. Over the next decade it would become the standard service rifle of the USMC infantry (USMC).

The Marine Corps did consider a Product Improvement Program (PIP) for the M16A4 as the M27 began replacing the M249. Colloquially called the”M16A5″, this modified M16A4 would utilize a a VLTOR Carbine length adjustable buttstock and a VLTOR or Daniel Defense free floated rail system. While this would have undoubtedly improved performance, the M27 was a superior system that would quickly win over the hearts and minds of Marine grunts and commanders alike. Much like the M16A2, as the M27 became standard issue for infantry Marines, the M16A4 would be used for training Marine recruits and students at the School of Infantry, as well as issued to support Marines. It would be in these roles that the M16A4 PIP would make its mark, as some M16A4s have been and are still issued with M4 Carbine lowers or stock systems.

The USMC would use various unique iterations of the M16A4 with a collapsible, M4 Carbine length buttstock system. Some of these variants were used operationally, including the Marine pictured bottom left in Iraq.

The USMC M16A4 Support / Training Rifle

Just like the M16A2, the M16A4 would continue its service life training new Marines and arming support units after the M27 was adopted as the new standard service rifle for the infantry. In 2010, the Marine Corps would begin the process of arming Marine recruits with ACOG equipped M16A4s, cementing the evolution from iron sights. As the M16A4 setlled further into this rile, more would be equipped with M4 Carbine collapsible buttstocks, primarily to accommodate shorter support Marines and recruits. While the sun has set on the M16 as a frontline service rifle, it still continues to play a critical role to this day, proving the incredible adaptability of the Marine Corps’ longest serving rifle.