6.5×42mm

The 6.5×40mmTK, also known as .264 Rjinders in Tiperyn service and 6.5mm NSC colloquially, is a, designed by Tiperyn and Koryeo firearms engineers in the early 1950s. The cartridge is, composed of an aluminium core with a copper jacket running approximately 2/3rds the length of the bullet.

The 6.5x40mmTK was developed following early experiences during the Kesh Wars. Although the Grand Campaigns from 1915 to 1925 had shown that most engagements were between 300 m to 600 m in distance - leading to the adoption of the 6.5x35mm Short intermediate cartridge - fighting in Kesh thus far had involved heavy fighting in both mountainous terrain and urban environments. By 1955, troops of the Tiperyn Realm Defence had requested a cartridge that would be more effective at longer ranges than 6.5x35mm Short, as well as more controllable in close quarter environments and lighter than. The development of a replacement cartridge began in that year under the auspices of the North-South Defence Consortium and by 1957, 6.5x40mmTK entered military trials.

The cartridge is currently the standard and  cartridge of the North-South Concordant. Some nations field it in limited roles as a cartridge, including a number of Tiperyn light infantry units. However, due to its lack of penetrating abilities, coming down to its low density aluminium core, most retain larger full-power rifle cartridges in this role.

Design
The novelty and utility of 6.5x40mmTK comes down to its aluminium core and partially copper jacketed bullet. It was determined through early trials that a conventional lead core and fully copper jacketed cartridge could not achieve the standards laid out by the Tiperyn and Koryeo governments. Among these were:
 * 1) Effective aimed fire against a point target in excess of 1,000 meters.
 * 2) Comparable controllability in fully automatic fire as the 6.5x35mm Short cartridge and superior recoil performance to 6.5x55mm.
 * 3) Size and weight requirements that made it reasonable for a soldier to carry at least 150 rounds of ammunition in magazines on their person. At the time, Tiperyn soldiers' standard combat loading was 150 rounds of 6.5x35mm Short (6x 25-round magazines) or 90 rounds of 6.5x55mm (9x 10-round magazines).

A bullet diameter of 6.7mm was chosen because of previous experience with 6.5x35mm Short and 6.5x55mm, as well as its superior, or the bullet's ability to overcome air resistance. The aluminium core was chosen to lengthen the projectile, leading increased ballistic performance while retaining a lower mass and, thus, lower recoil energy. As a result, in addition to having a higher muzzle velocity than its peer cartridges, it was able to retain its velocity further out, remaining stable and past 1,000 meters. However, in early tests, it was found that an all aluminium projectile did not engage the rifling as effectively as copper jacketed projectiles. Thus, after leaving the muzzle, the projectiles would not stabilize themselves effectively and cause unsatisfactory accuracy deviations. A copper jacket was added about 2/3rds the length the bullet, including at its widest diameter to better engage the rifling. Additionally, the denser jacket allowed the projectile to retain much more of its rotational inertia about its axis, allowing for better stabilization.

The case was derived from the 6.5x35mm Short case - itself a shortened version of the 6.5x55mm case - length to allow for a larger powder charge for the larger projectile.

The result was a cartridge that met most of the requirements set forth during trials. The estimated recoil energy of the 6.5x40mmTK fired from 3.18 kg (7 lbs) rifle is 9.44 J (6.96 ft/pounds) and 6.48 J (4.78 ft/pounds) from a 4.6 kg (10.2 lbs) squad automatic weapon. For comparison the recoil energies of and  fired from 3.18 kg rifles are 6.86 J and 11.47 J respectively. Meanwhile, 6.5x40mmTK outperforms these cartridges in effective range against a point target by 500 meters to 650 meters.

Drawbacks include bulkier magazines, although the cartridge itself is comparable in weight to. Additionally, because it makes use of a low density aluminium core, 6.5x40mmTK has poor performance penetrating barriers and hard targets. However, this also means there is a decreased chance of in urban environments. The projectile typically on impact, decreasing its effectiveness against walls and body armor, but creating more severe wound channels in soft targets. As the North-South Concordant has largely been engaged in counter-insurgency operations against soft targets in Kesh since the 1950s, the tradeoff has been judged to be acceptable.

External ballistics
Standard military standard 6.5x40mmTK was designed to retain much of its velocity over long distances. With a 66.2 grain (4 gram) projectile, 6.5x40mmTK remains above the transonic range (Mach 0.8 to 1.2). This improves overall stability, as air flow is disturbed once objects enter the transonic range.

Users

 * Koryeo: Adopted in 1958. Currently fielded as an assault rifle and squad automatic weapon cartridge.
 * Naseristan: Adopted in 1990. Currently in limited use by militia special forces.
 * Thesparos: Adopted in 2001. Currently fielded as an assault rifle and squad automatic weapon cartridge.
 * Tiperyn: Adopted in 1958 to replace 6.5x35mm Short and 6.5x55mm as the cartridge chambered by its standard issue service rifles. Currently fielded primarily as an assault rifle and squad automatic weapon cartridge, with limited service as a designated marksman and general purpose machine gun cartridge in the Fleet Marine Corps.