The Case for a New Military Caliber

The case for a new military caliber is a popular but controversial topic that arouses interest and emotion out of all proportion to the importance of small arms within the Army’s overall portfolio of weapons.

But, as the US Army begins to formulate a requirement for its Next Generation Small Arms (NGSA) Program and re-adopts 7.62 mm weapons more widely as an interim measure, it may be relevant to revisit this topic, not least because the UK may decide to adopt whatever new system America chooses.

Perhaps the best place to start any discussion about a new military capability is with the requirement. The focus here is platoon and section-level small arms and the key performance criteria are range and target effect. Prior to the UK adoption of 5.56 mm, British Army small arms doctrine required riflemen to engage targets individually to a range of 300 metres; sections firing collectively to 600 metres; and light machine guns to 800 metres. Target effect or lethality was defined by the ability to penetrate a steel helmet at the above ranges. Between 1888 and 1986, these requirements were comfortably exceeded by Britain’s .303 No.4 Rifle and Bren Light Machine Gun; and later, by the L1A1 Self-Loading Rifle and L7A2 General-Purpose Machine Gun, when 7.62×51 mm ammunition became the first NATO standard caliber.

In 1963, only a few years after 7.62 mm NATO was adopted, the US Army fielded a new rifle, the Armalite M-16, that fired a radically smaller ammunition type: 5.56×45 mm. The decision to adopt it was driven by a belief that 7.62 mm was much more powerful than really necessary. Inspired by two small arms studies, the Hall and Hitchman reports, published in 1952, the Small Caliber High Velocity (SCHV) concept was based on two assumptions: first, that 90{4dab693c107f7b6d4058a0febcf4eed43717abc6a37e80004208d6080fd302b5} of all infantry small arms engagements took place at ranges of less than 300 metres; second, that a smaller caliber projectile in a lighter cartridge case would allow a soldier to carry more ammunition for a given weight. The benefit of more rounds carried was the ability to deliver more rounds on target increasing the probability of a hit.

Besides ammunition weight reduction, the other benefits of a smaller caliber were reduced recoil and reduced weapon weight. These made a rifle easier to control helping soldiers to shoot more accurately. Compared to 7.62 mm, a 5.56 mm cartridge weighs half as much, 12 grams instead of 24 grams. Bullet weight is 4 grams for 5.56 mm versus 9.5 grams for 7.62 mm. Recoil energy for 5.56 mm ammunition is 4.7 Joules, whereas with 7.62 mm it is a whopping 16.8 Joules.

The potential downside of a small bullet is insufficient lethality. However, ballistic research shows that projectile velocity and energy are important determinants of terminal effectiveness. Even very small artillery fragments can kill when they hit a vulnerable part of the body at high enough velocities. Moreover, SCHV projectiles are designed to yaw or upset in soft tissue, which means that the bullet tumbles forward upon penetration to inflict a more serious wound than a bullet that travels straight through. The proponents of small calibers further emphasised that, regardless of caliber, shot placement is the key factor that ensures target effect. This depends on training as much as the weapon or ammunition.

Despite early teething problems, overall feedback from usage in Vietnam suggested that the US Army’s new M193 5.56 mm cartridge was a promising development. Although the small bullet did not always behave consistently and could be deflected by thick foliage, the SCHV concept proved popular with US troops because of the weight savings it provided. Over a ten-year period, weapons and ammunition were refined to unlock further potential. In 1979, NATO held a second caliber competition to select a SCHV round. The winning Belgian 5.56 mm SS109 / M855 design was able to penetrate a NATO steel helmet at 500 metres. It was also optimised for use in light machine guns. Notwithstanding the improved performance on offer, it was still necessary to retain 7.62 mm for long-range machine guns, sniper applications and company-level weapons.

Apart from a few brief skirmishes, the new 5.56 mm NATO caliber was not widely used in combat until 2002. However, soon after US troops began to deploy in force to Iraq, reports began to surface of inconsistent lethality, plus range and accuracy problems. An investigation showed that the primary cause was the US Army’s adoption of the M-4 carbine. This had a 14.5” barrel instead of the longer 20” length of the previous M-16 rifle. The shorter barrel reduced the velocity and striking energy of the bullet.

In Afghanistan, units frequently found themselves overmatched by Taliban insurgents using full-power 7.62×54 mm Russian and 7.62×51 mm NATO weapons and ammunition. Typically, the enemy would open fire at ranges above 600 metres. While such fire lacked precision, it was used to canalise troops into IED kill zones. Any squad equipped with only 5.56 mm weapons would not be able to return effective fire. In some instances, 5.56 mm was not effective beyond 200 metres. In particular, troops equipped with the FN 5.56 mm Minimi Para LMG, which also had a short barrel, found it to be inadequate.

Such problems led to the widespread re-adoption of 7.62 mm machine guns and the acquisition of 7.62 mm designated marksman or sharpshooter rifles.  The USA, UK, Germany and other NATO Alliance members also decided to develop improved 5.56 mm loadings. With increased range and lethality, 5.56 mm is now considered to be effective to 400 metres, or 300 metres from short barrel weapons.

In 2009, UK analysis of combat engagements in Afghanistan showed that more than 50{4dab693c107f7b6d4058a0febcf4eed43717abc6a37e80004208d6080fd302b5} took place at ranges above 300 metres. Although a great many occurred at less than 75 metres, the need to engage enemies beyond 500-600 metres seemed to question the earlier findings of Hall and Hitchman. Those who advocated the adoption of a smaller caliber also failed to consider the importance of suppression. With a reduced signature and striking effect, 5.56 mm simply did not suppress as well as larger calibers, especially at longer ranges.

Another factor that feeds the discussion is that optical gun sights are now fitted almost universally to infantry weapons. Shooting accurately beyond 500 metres has become less difficult than it used to be. UK soldiers in Afghanistan equipped the 7.62 mm L129A1 Sharpshooter rifle found that they could routinely hit targets at 800 metres. As optical sights gain fire control systems with integral laser range finders and ballistic computers, the ability to neutralise targets at longer ranges will improve.

After more than a decade of continued combat use in Iraq and Afghanistan, the small arms inventories of most NATO armies are now worn-out and need replacing. The key question is what is the future requirement, and whether it encompasses the need for infantry squads to engage targets at 600 metres and beyond? With the future operating environment characterised by urban, littoral and other complex terrains, perhaps 300 metres is sufficient?

Germany, France, Australia, and New Zealand have already begun the process of adopting new small arms. Most nations are adopting a mix of 5.56 mm and 7.62 mm weapons with the smaller caliber used in rifles for short-range work and the larger caliber used in machine guns and sharpshooter rifles for long-range work. However, another important consideration is the need to reduce the infantry soldier’s combat load. While 7.62 mm weapons overcome range issues, re-adopting 7.62 mm weapons has increased the weight of ammunition that needs to be carried. It also imposes a greater logistical burden. For these reasons, the USA is going down a different path. It is evaluating alternate calibers to see whether a single intermediate cartridge, between 5.56 mm and 7.62 mm, can replace both at platoon and section-level.

Over the years, many armies have conducted caliber studies. Almost all have concluded that the ideal military caliber to deliver 600-800 metre performance lies between 6 mm and 7 mm. Some countries have gone a stage further and developed intermediate caliber prototype ammunition including the following:

• .276 (6.8 mm) Pedersen (USA)
• 6.5 mm Arisaka (Japan)
• 6.5 mm Carcano (Italy)
• .270 (6.8 mm) Enfield (UK)
• .280 (7 mm) Enfield (UK)
• 6.25 mm Enfield (UK)
• 6 mm SAW (USA)
• 6.5 mm Grendel (USA)
• 6.5 mm HVAP (USA)

Modern intermediate caliber options deliver increased range in a smaller package by having bullets that are more aerodynamically efficient. Such projectiles lose energy less quickly than legacy ammunition types. Whatever is adopted, the challenge is to increase range and target effect without substantially increasing ammunition size, weight and recoil.

If a new caliber is too close to 7.62 mm in size, shape and performance, it may be simpler to stick with 7.62 mm. If, on the other hand, it is not significantly more capable than existing 5.56 mm ammunition, then it will not be able to substitute 7.62 mm as well.

As the debate continues, a new requirement has emerged. This is the need to defeat advanced Level IV ceramic body armours. It may well drive NATO armies to re-adopt 7.62 mm or to acquire a new caliber that is more powerful than 5.56 mm. Tungsten bullets could solve the problem, but are expensive.

As cased-telescoped cartridge technology matures and steel, polymer and other lightweight versions of legacy brass cases emerge, a larger caliber cartridge could be made to weigh only marginally more than an existing 5.56 mm brass cartridge.

To validate the many assumptions about intermediate cartridges, the US Army and US SOCOM are evaluating several 6.5 mm cartridges. It would certainly be worth the UK developing alternate caliber prototypes if only to build a robust fact-base that informs future choices. Whatever is finally selected, let’s hope it finally give infantry soldiers what they need, at least until a phased plasma weapon in the 40-watt range is available.

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