In Mark Bowden’s book, Black Hawk Down, he describes an incident where a soldier’s CAR-15 5.56mm carbine failed to put down an enemy combatant despite multiple hits to center mass. The ammunition used by the soldier was the M855 “green-tip” projectile, adopted by the U.S. Army and Marines in the 1980s.

Further reports of M855 underperforming lead the U.S. military on a hunt for a new cartridge that would be more effective. Surprisingly, the Army and Marines diverged in their approach, and for nearly half a decade both services fielded different cartridges as standard issue to their troops until Congress forced the Marines to use the Army round.

But which round is better? Have other NATO countries kept up with these innovations and developed “third-generation” 5.56 rounds of their own? Why did the Marines resist the change for so long?

To understand the problems with the M855, it’s worth looking at the round it replaced, the M193. The M193 was the cartridge the original M16 was designed for. It uses a fifty-five grain lead projectile with a copper jacket. It is designed to be stabilized by the 1:12 twist and twenty inches of the M16’s original barrel. The primary wounding mechanism of this projectile is yawing and fragmenting upon impact with flesh, both of which create a larger wound cavity than simply passing through.

The M193 was also adopted by a variety of other nations. Almost every nation that used the original M16 or M16A1, from the Republic of Korea to the United Kingdom utilized M193. France also adopted a 5.56mm round similar to the M193 in 1979, which also used a fifty-five grain projectile in a 1:12 twist barrel. However, due to the violent nature of the French rifles’ actions, the French cartridge utilized a steel case instead of brass.

But the M193 was not a NATO standard. In 1970, noting that the United States had diverged from the earlier 7.62x51mm standard cartridge, NATO wanted to re-standardize a single type of small arms ammunition and conducted their own series of tests from 1973 to 1979 with a variety of smaller “intermediate” cartridges in the same power range as the M193.

The trials involved firing at steel helmets at extended range, which caused rounds to be optimized for long-range ballistics and penetration on steel rather than terminal ballistics on soft targets.

The competitors were a British 4.85mm round, a German 4.7mm caseless round, and two variants of 5.56, one from America and one from Belgium. The Belgian round, the SS109, won the trials and was designated as the standard NATO infantry rifle cartridge under NATO Standardization Agreement (STANAG) 4172 in 1980. Most NATO nations adopted the SS109 as their first 5.56 round. The United States adopted it as the M855, painting the tip green to allow soldiers to easily differentiate it from older M193 ammunition.

The M855 uses a sixty-two grain projectile stabilized by a 1:7 twist barrel. The slower twist rate is required to stabilize the heavier projectile. The projectile has an enclosed steel penetrator that gives it superior penetration against steel. The round fragments upon hitting a soft target, but it requires a significant amount of velocity to do so.

The round was paired with the new M16A2 rifle, which combined a barrel that used the new twist rate with various other changes. The old M193 ammo was not backwards compatible with the M16A2, as the lighter ammo would not be stabilized and would yaw significantly in flight leading to poor accuracy.

The M16A2 and M855 combination was used by the U.S. Army and the U.S. Marines through the 1980s and into the 1990s. But during the 1990s the U.S. military began purchasing short barrel 5.56 carbines again, following poor experiences by special operations forces trying to use 9mm submachine guns in conventional direct action raids.

These carbines had shorter barrels, usually around 14.5 or sixteen inches, which gave them lower muzzle velocity compared to the full twenty-inch barrel of the M16. The lower velocity of these carbines led to incidents such as those in Mogadishu where the M855 fired out of a CAR-15 failed to fragment and put targets down reliably.

The issue of M855 lethality continued to plague the military in the post-9/11 era. As the wars in Iraq and Afghanistan dragged on, the U.S. Army and Marines began to issue out M4 Carbines, which use a 14.5-inch barrel, in larger numbers. This only exacerbated the problem with M855.

Special Operations Forces were already well aware of the limitations of M855 in short 5.56 rifles following their experiences in Mogadishu. They already had an alternate bullet in the form of the Mk262, a seventy-seven grain open tip match round originally issued with the Mk12 SPR, a version of the M16 designed for precision fire.

The Mk262 offered better terminal performance in flesh and ballistic performance compared to M855, even when fired from short carbines. Early in the post 9/11 era, most SOCOM units switched to the Mk262 round for all their 5.56 weapons. The USMC also ordered Mk262 in limited quantities in the mid-2000s.

But SOF wanted something better than the Mk262. The division of the Navy responsible for their small arms programs, Naval Surface Warfare Center Crane Division (NSWC Crane) began development of a new round in 2005 to address user concerns with the round.

This new round reverted back to the M855’s standard sixty-two grain weight and was designed primarily for better terminal performance and the ability to be “barrier-blind”: to shoot through soft cover like thin walls and car doors without significant trajectory change. It featured a large solid copper core behind the lead filling. The open tip was retained from the Mk262 for accuracy.

This round was called Mk318 Special Operations Science and Technology (SOST). It was adopted by SOCOM in 2007, replacing the use of Mk262 in most roles. The USMC adopted the Mk318 in 2010. At the time it was not to fully replace the M855, but soon afterwards the Mk318 became the de-facto standard issue round in the USMC for Marines heading into combat. The M855 was relegated to training use.

The Army embarked on their own program in the 2000s to field a new round. The Army was already developing a new round, called M855A1, at the time. Early on, the primary objective of the M855A1 was to develop a more environmentally friendly round by eliminating the use of lead in the projectile. But problems with the M855’s lethality lead to the addition of a lethality requirement to the M855A1 project.

The military, under the auspices of the Joint Services Wound Ballistics Integrated Product Team (JSWB IPT), did a significant amount of research into determining why the M855 performed so poorly in terminal ballistics. They found that most bullets would yaw in flight to some degree or another, and the fragmentation performance of a bullet would often be affected by what angle the bullet had when it hit the target. Some M855 hits at high yaw would perform excellently, fragmenting and yawing within a few inches of ballistic gelatin. Others would go significantly further before exhibiting these effects.

JSWB IPT concluded that in order to achieve consistent improved lethality, the new round needed to have consistent fragmentation and yaw performance regardless of the impact angle. The new round was also designed to fragment reliably within a greater range of velocities, instead of just the high velocities required by the SS109.

The round design for the M855A1 was finalized in 2010. The end product is a sixty-two grain projectile that has an exposed steel-tipped penetrator followed by a copper core, totally eliminating any lead in the bullet. The powder burn time in the cartridge was also optimized for the Army’s shorter M4 Carbines and chamber pressures were increased to achieve similar velocities to the M16 out of the M4’s shorter barrel.

The M855A1 is barrier blind, has better and more consistent terminal effects, and achieves better penetration against body armor than any earlier general purpose round (including the Mk262 and Mk318). It even outperforms the older M855 against steel armor and shoots flatter than earlier rounds.

However, the round didn’t work well in standard magazines. The steel tip of the M855A1’s exposed penetrator would dig into, scratch, and potentially chip the feed ramps of M4 Carbines and M16s when fed from standard-issue grey aluminum magazines.

To fix this the Army designed a new, flat dark earth aluminum magazine called the Enhanced Performance Magazine (EPM) that changes the feed angle to reduce wear when using M855A1. The Army adopted this magazine in 2016; however, the Marines did not.

The Marines opted for Magpul’s plastic PMAG Gen 3 magazine instead. PMAGs have seen extensive use with Special Operations Forces and even in some regular Army and Marine units since their creation in 2007. The PMAG was already designed with an altered feed angle to increase reliability, so it feeds M855A1 without issue.

But while the Marines adopted the PMAG in 2016, they still resisted the change to the Army’s M855A1 round. The Marines originally planned to adopt the M855A1 in 2009, when the Army was finalizing its design work on the round, but problems with the accuracy of the trajectory of the round caused the Marines to pull out of that commitment. Instead, they bought the Mk318.