6.5×42mm

The 6.5×42mm (official Tiperyn nomenclature .256 Rjinders) round is a rimless bottlenecked family developed in the the early 1950s as part of a jointly developed project between Tiperyn arms manufacturer Rjinders Amorers and Koryese firm Song Arsenal.

Development began on a new rifle cartridge to replace older full-power rifle cartridge in Tiperyn and Koryese service began in the late 1940s, but was accelerated at the onset of the Great Kesh War. Due to competing design requirements from the two nations, the round would have to balance two paradoxical characteristics: controllability in full-auto for at distances below 400 meters and the ability to maintain a relatively flat trajectory out to at least 1,000 meters. The joint Rjinders-Song submitted 6.5×42mm was among the first rounds developed that took into consideration bullet shape to reduce drag and used the relatively exotic but low density aluminium to reduce recoil impulse.

The 6.5×42mm round achieved the original design requirements through the use of a long, low-drag bullet profile to reduce drag and a copper semi-jacket to increase stability and better engage with barrel rifling. As an intermediate cartridge, it was much lighter, shorter, and softer shooting than the rifle cartridges it replaced in addition to having superior bullet drop characteristics at longer ranges. Due to its low cross section density, though, owing to its use of low density materials, 6.5×42mm is more susceptible to wind and the round is lacking when it comes to barrier penetration. Heavier lead-core, full metal jacket variants have been introduced for use in purpose-built subsonic suppressed carbines specifically, although these rounds are not always able to cycle in the standard delayed blowback firearms issued by the Tiperyn military without modification.

Design
The 6.5×42mm round is relatively long (at 68 mm overall length) and tapered compared to other contemporary intermediate cartridges, meaning magazines for rifles that chamber it are often wider and more curved — although not when compared to some full-power battle rifle cartridges. Its most distinguishing feature is the exposed aluminium tip partially jacketed by copper.

The overall design and rationale behind 6.5×42mm is relatively esoteric. The aluminium core was chosen over conventional lead due to its low density, meaning mass for a given volume could be reduced, in turn reducing the recoil impulse when compared to an equivalently sized round. Other lower density materials were considered, such as plastics or steel, but aluminium was the material selected by the Rjinders-Song team to submit to field trials. The copper semi-jacket was added to give the rifling a more malleable surface to engage and by shifting the weight to the outside of the bullet to increase rotational stability. Because of the round's low cross sectional density — a side effect of using aluminium — standard aluminium-cored 6.5×42mm requires a high rifling twist of 1 in 5 inches to be adequately stable. The combination of a high muzzle velocity (at 950 m/s out of a standard 410 mm barrel), high twist rate, and low-drag spitzer-boatail bullet profile compensates somewhat for the low-cross sectional density, giving it a superior trajectory in terms of bullet drop when compared to other intermediate or even full-power rifle cartridges. However, at the same time it is more susceptible to windage than denser rounds and because of the heavy use of aluminium is ineffective at penetrating barriers or stronger body armors. However, these factors were not the primary concerns laid out in the original tender requirements and 6.5×42mm was ultimately found to be adequately controllable in full-automatic and burst fire as well as able to repeatably hit targets out to 1,000 meters in rifles equipped with telescopic sights.

External ballistics
Standard military standard 6.5×42mm was designed to retain much of its velocity over long distances. The cartridge's standard 74 grain (4.8 gram) projectile remains above the transonic range (Mach 0.8 to 1.2) out to 1,000 meters. This improves overall stability, as air flow is disturbed once objects enter the transonic range.

Comparative statistics
Still, due to its low recoil impulse, long effective range, flat trajectory, and relatively compact form factor in comparison to full-power rifle cartridges, 6.5×42mm has found limited success as a designated marksman rifle cartridge in addition to service rifles and squad automatic weapons. Overall, 6.5×42mm fills an intermediate niche between the North-South Concordant's standard 6.5x55mm rifle cartridge and its 10×22mm pistol cartridge. Further, it more effectively fills the role of a general purpose cartridge due to its light weight and low recoil energy coupled with its good ballistic coefficient and long effective range. However, due to its use of primarily aluminium construction, the Concordant nations who have standardized on it have largely shied away from fielded 6.5×42mm in heavy machine guns, general-purpose machine guns, sniper rifles and vehicle-mounted weaponry in general.

GvP. 1956
Gevearpatroan 1956, abbreviated as '''GvP. 1956''' is the original variant of 6.5×42mm introduced into Tiperyn and Koryese service. It is used primarily in section-level weapons, including service rifles and light machine guns.

SkP. 1956/59
Skerpskuterspatroan 1956/58, abbreviated as '''SkP. 1956/59''' is a modified, specialist variant of the original GvP. 1956 designed for use in.

GvP. 1956/70
Gevearpatroan 1956/70, abbreviated as '''GvP. 1956/70''' is a subsonic, lead-core variant of 6.5×42mm used in suppressed carbines within special forces units and law enforcement agencies.

Users

 * Koryeo
 * Tiperyn - Issued as the standard issue rifle, light machine gun, and light designated marksman cartridge since 1956.

Former users

 * Veikaia