In today's assault rifles, reduced ballistic performance ammunition dominates. These so-called intermediate cartridges were created by reducing older, more powerful types, with the aim of obtaining "feed" compatible with the new generation of small-caliber and micro-caliber firearms (a micro-caliber rifle is one with a caliber under 6 mm).
For „black rifles“
In the Western world, the most well-known cartridge is the 5.56×45 mm NATO (or its civilian version, .223 Remington), the development of which was associated with the creation of AR-15 series firearms. In addition to the creator of the famous "black rifle" Eugene Stoner, Robert Hutton, the technical editor of Guns and Ammo magazine, played a key role in the development of ammunition. Both men based their work on the proven .222 Rem cartridge.
In 1964, the novelty became the standard ammunition of the US Army under the designation 5.56 mm Ball M193. The 55-grain (3.6 g) bullet reached a muzzle velocity of 990 m/s, and American soldiers tested it with their M16s in the Vietnamese jungle. War experiences and further development led to the emergence in the early 1980s of both a more modern rifling of American assault rifles and improvements in the ammunition itself. The modification came from the Belgian arms factory FN Herstal, whose designers aimed for greater accuracy. They replaced the previous rifling twist rate of 1:305 mm with a twist rate of 1:178 mm, increasing the bullet's spin.
In connection with this innovation, the improved 5.56mm M855 cartridge was born, with the SS109 bullet weighing 62 grains (4 g) leaving the muzzle at a velocity of 945 m/s. However, the muzzle energy increased from 1764 joules to 1786 joules. It is this cartridge that today forms the rifle standard in the armies of the North Atlantic Alliance. While it may no longer be sufficient for some tasks, attempts by developers have not yet produced a fully-fledged successor. Moreover, weapons of this caliber are so widely spread that replacing them with a different model - necessary when transitioning to new ammunition - would be enormously costly.
For a long time, the 5.56×45 mm caliber was accompanied by terrifying rumors regarding its effectiveness - for example, that a hit, even just to the arm or leg, would immediately incapacitate the enemy. This was due to the exceptional energy transferred upon impact. In reality, a .308 Winchester caliber bullet achieves nearly twice the energy, so it was a myth. However, it is true that despite its small caliber, the 5.56mm cartridge has a stronger effect on tissue than many expect - but instead of energy, it is caused by loss of stability upon contact with the target and bullet fragmentation.
Russian classic
The Soviet 7.62×39 mm cartridge was created during World War II for the SKS semi-automatic carbine - hence its nickname "model 43". Later, it was also used as "feed" for the legendary AK-47. The original version featured a bullet with a steel core covered with a lead jacket and a jacket made of steel plated with tombac. The bottle-shaped lacquered cartridge case contributes to more reliable function in automatic and semi-automatic rifles - it is responsible for the typical curved shape of magazines.
With a bullet weight of 122 grains (7.9 g), the projectile achieves a muzzle velocity of 710 m/s and a muzzle energy of 1991 joules. It was used by the armed forces of Warsaw Pact countries, including the Czechoslovak People's Army (ČSLA), whose members, however, never received Soviet rifles. This is because the design team led by Jiří Čermák came up with their own weapon for this caliber at the end of the 1950s. The famous "sickle" or SA vz. 58 was born, with several generations of conscripts shooting it during their military service.
It is worth noting that initially, the Russian cartridge introduced the M43 bullet, which was so stable when penetrating tissues that it caused relatively little damage ("mixing" within the wound began only after 30 cm). Therefore, in the 1960s, it was replaced by the longer M67 projectile, which destabilizes earlier due to a backward shift of the center of gravity upon impact. Sometimes it even passes through the target sideways, resulting in a very wide wound.
Too much power
It is interesting to note that the described cartridge structurally resembles the American 7.62×51 mm ammunition (the civilian version is labeled .308 Winchester), developed after World War II by the U.S. Army Ordnance Corps based on knowledge gained from the front. Experience showed that modern combat would be conducted more at medium and short distances, necessitating entirely new ammunition. However, the basis of the design was the old familiar .30-06 Springfield.
Winchester engineers initially experimented with a 7mm caliber but eventually focused on a 7.62mm caliber. The length of the cartridge also changed - from the original 47 to the final 49 mm. The result was introduced in 1950 and immediately began army testing. This led to the shortening of the cartridge case, and three years later, the novelty became the official ammunition of the North Atlantic Alliance as the 7.62×51 mm NATO. While American soldiers used the cartridge for shooting from M1A rifles, Springfield M14s, and M60 machine guns, in Europe, it was used - alongside civilian use - in FN FAL assault rifles and FN MAG machine guns.
As mentioned, in 1980, NATO representatives chose the 5.56 mm caliber as the standard. Among the main arguments was the unnecessarily large power of the 7.62mm ammunition for modern assault rifles, excessive recoil, and excessive weight, which meant that a soldier could carry less ammunition. Nevertheless, the original caliber continues to be used in machine guns and sniper rifles, where the substantial weight of the projectile and kinetic energy are advantageous. The Ball M80 version has a 150-grain (9.7 g) bullet, a velocity of 838 m/s, and a muzzle energy of 3405 joules, while the Ball M852 version has a 168-grain (10.9 g) bullet, a velocity of 811 m/s, and a muzzle energy of 3585 joules.
Innovation for the „fifteen“
Among the newest types of ammunition for assault rifles is the American 6.5mm Grendel (6.5×39 mm), which was created in 2003 by the design team of Arne Brennan, Bill Alexander, and Janne Pohjoispää. Their goal was to create a more effective cartridge specifically designed for the AR-15 compared to the .223 Rem ammunition. It was supposed to be a medium-powered type for shooting at targets at distances of 180-730 m.
The standardized chamber length of the mentioned rifles somewhat limited the designers, so they decided to use a shorter cartridge case with a larger diameter, which would accommodate more gunpowder. Ammunition is produced with several bullet weights, with the most common being a version weighing 130 grains (8 g), which leaves the barrel at a velocity of 770 m/s with a muzzle energy of 2465 joules.
The base of the cartridge case has the same diameter as the .220 Russian, 7.62×39 mm, or 6.5 mm PPC cartridges. However, compared to standard "black rifle" ammunition, this dimension is larger, which requires the use of a non-standard receiver and reduces capacity - a Grendel cartridge case fits only twenty-six rounds into a magazine designed for 30 rounds of 5.56 mm ammunition. Performance-wise, the 6.5mm Grendel falls somewhere between 5.56×45 mm NATO and 7.62×51 mm NATO. Its supporters appreciate the fact that, unlike both, it can deliver more energy to the target over longer distances. If a shooter wants to achieve further improved characteristics compared to the 7.62×51 mm with a Grendel, they must choose a rifle with a longer barrel and opt for a heavier version of the ammunition.
