12.7mm x 39mm
The
12.7mm x 39mm is essentially a 12.7 x 108mm cartridge cut down to a much smaller 39mm case, which holds less powder, and thus possesses less kinetic energy. In part due to the damage caused to brass of the 12.7mm x 108mm machine guns which made them unreloadable and a desire for a mid range machine gun to fullfill the gap between large rifle (such as the 7.62mm x 54mm) and heavy machine gun roles (such as the .50 cal), the experimental 12.7 x 108mm cartridge was born. Cutting down the case to below the damaged area of the ejected shell, the brass was refitted with the same round and powder, although being in the smaller shell was approximately 2.8 inches shorter, and possessed 8,000 joules, compared to roughly 20,000. Comparative cartridges, such as the 7.62mm x 54mmR, possessed roughly 3500 joules, while the .50 cal possess nearly 20,000. The 12.7mm x 39mm is thus an intermediate heavy machine gun cartridge, designed to full a very specific niche role.
Although the energy of the cartridge is approximately 40% of that of the full sized cartridge (largely due to the smaller cartridge case), it nonetheless is rather small. The shorter length of the cartridge has a number of advantages, more importantly that it allows feeding from similiar sized guns, such as the PKM, which rely more heavily on the length of the cartridge than the width. While not substantially lighter weight, the cartridge nonetheless is much smaller, and thus easier to feed and extract in relatively small machine guns. With the same pressure and less energy, a much larger, but still relatively light 35 pound machine (16 pounds for the PKM) is capable of firing the cartridge, making it a potential light crew served weapon. Finally, because the round relies on the effects of the high explosive armor piercing incendiary round (HEIAP), it's kinetic energy is less important to the overall damage of the round. For practical combat situations, it's considered to have sufficient energy and range. The weapon is almost always mediated to 300 RPM, and is fired almost like a sniper rifle, instead of a machine gun, in order to reduce the recoil and conserve on the incredibly heavy ammo. In comparison, a machine gunner could carry approximate 3-4 times the amount of ammunition of 7.62mm x 54mmR ammo. A machine gunner that carried 300 rounds might only be able to carry 75 rounds, instead, making ammo conservation much more important. It nonetheless fulfills a very specific niche role.
The primary purpose of these munitions is armor penetration, but unlike SLAP rounds (saboted light armor penetrator) which get their armor-piercing ability from the propulsion of a 7.62mm tungsten heavy alloy bullet from a 12.7mm barrel (.50 caliber) using a sabot with much more energy than is usually possible from a 7.62mm round,
HEIAP munitions rely on the high explosive power to destroy and penetrate armor. The initial collision ignites the incendiary material in the tip, triggering the detonation of the HE charge. The second (zirconium powder) incendiary charge will also ignite. This burns at a very high temperature, is not easily extinguished, burning at nearly 4,000 degrees. The remaining element of the round is the tungsten carbide penetrator; This has a large amount of kinetic energy and will penetrate the armor as a solid-cored armor-piercing shot would. This will take some of the incendiary material through the armor. The MK 211, from which the roundi s based on, claimed to penetrate up to two inches (51 mm) of rolled homogeneous steel armor.
