A Survey of Military 5.56

(Featured Image: M855A1 round. Note distinctive steel penetrator.)

Author’s notes: In terms of the scope of this article, despite the historical importance of the civilian realm of shooting on military cartridges, and the superlative performance of many civilian designs, this article will wherever practical limit itself to military rounds, primarily standard issue ball rounds, or their replacements. Next, with regards to measurements, preference will be given first to the units used by the original source, and then to whichever unit is the most confusing. Bullet weights will be given in grains, abbreviated “gr”. Barrel twists will be given as 1:N, to indicate that there is one complete twist of rifling for every N inches of barrel. Lastly, I have divided the cartridges into informal generations, partially for convenience, partially to illustrate overarching design trends. This is the artifice of a layman, and while I think it is beneficial, it is by no means rigorous, nor has it been subjected to any level of criticism, which I happily invite.

First Generation

Development of .223, 1:14 becomes 1:12
As outlined in limited detail in my earlier piece “Green Jungle, Black Rifle”, the .223 round was developed alongside the AR-15 in the late 1950s and early ‘60s. The middle ground between two prior test cartridges in .22 caliber, a 68gr homologue of .30 M1 Ball and a 41gr offshoot of .30 Carbine, was proposed to the Chief of Ordinance in 1955 with a bullet weight of 55gr, and the earliest AR-15s were chambered in this “.222 Special”. As the design matured, cartridge development was done in part by Remington, who in 1959 changed the name permanently to .223 Remington. Originally this was paired with a 1:14 rifling, which was used successfully on .22 caliber “varmint” rifles, however, US Air Force testing in 1963 showed that this did not impart sufficient spin on the bullet to stabilize it in subzero temperatures, leading to problems with accuracy. Following approval from Secretary of Defense McNamara in July ‘63, the AR-15 was standardized on a 1:12 twist.

M193
Consisting of a 55gr lead alloy boat tail FMJ round, M193 is almost entirely dependent on its velocity for terminal ballistics. Although its velocity drops off with range more rapidly than modern, heavier bullets, the high velocity of M193 in full length barrels can produce impressive fragmentation and armor penetration at close range. Homologues include the South African M1A3, British L2A1, and South Korean KM193.

French F1, F1A and PPA
An unusual breed of 55gr 5.56 bullets belongs to the French Army, who adopted the lever-delayed blowback FAMAS as their standard service rifle the late 1970s. Following the pattern of the M16, FAMAS F1 rifle use a 1:12 twist barrel, limiting them to 55gr bullets. In addition, the blowback operation of the FAMAS means that brass cased ammunition suffers over-pressure and case ruptures, and thus all French production ammunition for these rifles is steel cased. Three primary types are operated, the F1 ball round equivalent to M193, the F1A tracer round, and a PPA or “Pouvoir Perforant Amelioré” armor penetrating round.

Second Generation

NATO standardization and the 1:7
In the years following the Vietnam War, NATO sought to standardize on a new small arms round amongst its member nations. Although many members used M193 pattern rounds, a variety of new concepts were fielded, including the British 4.85x49mm, the German caseless 4.7x21mm, and fatefully, the Belgian SS109 5.56x45mm round. Two aspects of performance were focused on during the trials, that of long range, and of armor penetration, and in this the SS109 round won out. Its 62gr construction required a faster rifling twist than M193, at least 1:9, to properly stabilize. However, a crucial counterpart to ball ammo is tracer ammunition, especially when used by the light machineguns of a rifle squad. The slightly heavier and longer 64gr M856/SS110 requires a twist rate of 1:7 to properly stabilize, and this twist rate has been standardized on all US 5.56 weapons following the M16A2, though many NATO rifles use 1:9 instead. Although not a deliberate design consideration at the time, the 1:7 twist has proven to have a great deal of potential for growth, as it can adequately stabilize bullets as heavy as 77gr.

(M193 Left, M855 Right. Note lead slug on both, with M855 having a steel penetrator.)

SS109/M855
Adopted as NATO’s second standardized rifle caliber under STANAG 4172, SS109 or M855 “green tip” is a 62gr semi-armor piercing ball round, consisting of a steel penetrator and lead core surrounded by a copper jacket. Armor penetration range and velocity retention is improved compared to M193. Wounding on soft targets is accomplished primarily through yawing and tumbling as the bullet strikes the target. Homologues include the Canadian C77, German DM11, South Korean KM100, Australian F1, and British L15A1.

French Modernizations
With the NATO standardization on 62gr bullets which are incompatible with the FAMAS F1, a newer version, the G2, was developed in 1994. Equipped with a 1:9 barrel capable of stabilizing SS109 homologues, the G2 was adopted solely by the French Navy for use by its Marine and Commando units. Ball and tracer ammo, designated ON and TN respectively, remain steel cased.

Swiss GP90
Formally designated as the 5.6×45 Gewehr Patrone 90, this 63gr round is optimized for the unique 1:10 twist of the SG-550 for greater accuracy compared to the SS109 from which it was derived.

Third Generation

Failings of M855
With the dawn of the 21st Century came a shift in threats facing the 5.56 rifleman. No longer was he expected to hold the line at the Fulda Gap; COIN and counter-terror became his trade. His rifle changed as well; the US Army began to issue shorter barreled M4 Carbines in the place of M16A2 rifles. However, his cartridge remained unchanged, and soon reports began to surface from US troops in Afghanistan and Iraq of M855 failing to incapacitate the enemy, and “ice-picking”, where the rounds would simply punch neat .22 caliber holes in the target without depositing their energy or fragmenting.

To briefly address this issue, there are several factors at play. First, 5.56 as a whole and particularly M855 is velocity dependent for its wounding. Ideally, the bullet will either tumble and or fragment to achieve its terminal effects in a human. Increased use of optical sights means targets are being struck at longer distances, after a great deal of velocity has been lost, and the transition to carbines means that rounds will have less velocity to begin with. This is especially the case with M855, which was optimized for the full length barrels of the FN Minimi and M16A2. Secondly, the angle of attack at which a round strikes the target can dramatically change terminal effects at close range, and this was found to vary wildly even within the same lot of M855.

Lastly, the design of M855 was tailored for performance against armor at long range, not short range soft targets. Performance was specifically measured by the penetration of NATO plates and steel helmets at distance, something at which M855 excels. It was not, however, designed to meet the demands being asked of it, and it is thus reasonable to expect new designs to emerge to fulfill these roles, and it is to these that we now turn.

(M855 left, M855A1 right. Note again the steel penetrator above a copper core.)

M855A1
An extremely controversial round, M855A1 is a combination of two separate strands of development, that of a lead free “environmentally friendly” bullet, and the US Army’s desire to field a round superior to M855. The initial design was formulated in late 2007, and design work continued until June 2010. The 62gr round consists a stacked steel penetrator overtop of a copper core. The round has a higher chamber pressure, and therefore puts more wear on the rifle than M855, and requires a new magazine to avoid premature wear of rifles’ feed lips. In return, the round is more accurate, has armor penetration exceeding common 7.62 NATO, barrier-blind properties, and consistently improved terminal effects. Particularly, the issue of “ice-picking” is almost completely resolved, as the round fragments within mere inches of entering a soft target.

(MK318 cross section showing the Open Tip Match construction)

MK318
Separate from the development of M855A1, MK318 began as a cooperative project between NSWC Crane and Federal Cartridge Company in 2005 to produce an improved 62gr 5.56 cartridge. Its “Open Tip Match” construction aids accuracy, and combined with a rear penetrator provides barrier-blind performance. In addition, the OTM provides increased terminal effects via fragmentation in soft targets similar to, but legally distinct from the mechanism of hollow-point rounds, and provides wounding independent of velocity and yaw. Oft referred to as the Special Operations Science and Technology or SOST round, the round began service with SOF in late 2007, and began complimenting M855 in Marine use in January of 2010.

At time of this writing, the USMC and US Army use MK318 and M855A1, respectively. The 2017 National Defense Appropriations Act requires them to standardize on a single round by 2018, but no decision on which round will be chosen is forthcoming.

(MK262 Cross Section, again showing its OTM construction)

MK262
Originally developed for the MK12 SPR in 1999, MK262 is an adaptation of the 77gr Sierra MatchKing by Black Hills. Often used in civilian marksmanship competitions where precision at a distance is prized, this heavier round offers increased accuracy and longer range than M855, and its original application was in long-range marksmanship. However, it was realized that the OTM construction offered superior terminal ballistics over M855, especially with shortened barrels, and the round has become extremely popular with SOF units, and saw limited use with the USMC prior to the MK318’s development.

Australian F1A1
Developed circa 2011, the F1A1 round is a modification of the SS109 round, with an optimized bullet and case design to increase accuracy and reduce the effects of temperature variance on the cartridge. Armor penetration and terminal ballistics should remain unchanged relative to M855.

(L31A1 EP vs SS109 pattern round, credit The Register)

British L31A1
Similarly to the M855A1, the British MoD began in August 2016 to procure a lead-free ball round to replace SS109. Designated the L31A1 Enhanced Performance Ball, this round replaces the SS109’s steel core and lead base with a single hardened steel core. While this retains the overall shape and 62gr weight, armor penetration is vastly increased.

 

Finally, if the reader would be so kind as to indulge the soapbox, a brief word on the future of 5.56. American advances in bullet design offer an illustrative path forward for countries seeking to improve the lethality of the individual rifleman. M855, while certainly effective in increasing the effective range of 5.56, is outdated. M855A1 offers an excellent mix of penetration, barrier blind properties, and terminal effectiveness against soft targets, that is to say, people. MK318 offers perhaps the best mixture of barrier-blind properties and terminal effectiveness for carbine applications, and MK262 can increase the range of shoulder rifles to 800m, especially with advances in the accurizing of rifles and combat optics as a whole. While none of these options are perfect, 5.56 remains the best option moving forward for the individual soldier’s rifle cartridge. Suggestions to replace it with a larger caliber such as 6.8 or 6.5 are misguided, and completely at odds with the realities of the advances in bullet design. The light weight, low recoil, increased accuracy, longer range, and improved terminal ballistics of modern 5.56 offer the infantryman an unprecedented level of lethality and down range effect.