M6S115

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The M6S115 is a semi-automatic pistol of the  that is manufactured by. It is essentially a variant of the chambered for 11.5×34mm cartridge instead of the larger  cartridge and is used almost exclusively by the.

Design details
The M6S115 is short recoil operated, using the recoil of the previous shot to eject the spent case while loading the next round into the chamber. Like the, the M6S115 possesses an integral - system, simultaneously reducing both recoil and the sound of its gunfire. The pistol uses the same extended 20-round double-stack detachable box magazines that also feed into the M6I.

Ammunition
Unlike many of the other pistols of the M6 series, the M6S115 is chambered for a smaller, less-powerful 11.5×34mm cartridge, as opposed to the cartridge. The 11.5×34mm is noticeably weaker than the 12.7×40mm, but possesses significantly less recoil, allowing the M6S115 to be more controllable in automatic fire. Regardless, as a magnum cartridge, the 11.5×34mm is quite powerful itself.

SAP HP
The (SAP HP) round is of solid copper and is machined to have a fluted nose geometry, which not only aids the round in penetration, but also allows it to deliver almost all of its energy into the intended target.

SAP HE
The (SAP HE) round is made mostly of phosphor bronze, with a cavity filled with a stable explosive. The hard, phosphor bronze layer enables the round to penetrate body armor and enter the target, but is structured to deform shortly after entering in order to prevent the round from exiting the body and causing accidental casualties on the other side of the target. The explosive is then ignited by a fuse, detonating the entire projectile inside the target's body, producing a devastating wound that is crippling if not outright lethal. Compared to its SAP HP counterpart, the 11.5×34mm SAP HE sacrifices some ability to penetrate armor and other barriers in order to deal greater internal damage. Each SAP HE round is labeled with a yellow-painted tip. While the SAP HE round is the most common 11.5×34mm load used by the, it is generally not loaded in the M6S115s used by the.

HEC
Another, less common military cartridge variant is high-explosive concussive (HEC). Unlike the SAP HE round, the HEC only possesses a copper jacket with more explosive filling, which is detonated by an incendiary material at the tip that is first ignited by the round's impact. Instead of penetrating the surface and then exploding inside the target, an HEC round creates a larger explosion at the target's surface, releasing a shockwave that stuns and disorients the target, should he/she/it survive the shot. Combined with the low recoil and high rate of fire of the M6S115, HEC rounds are capable of pinning down heavier infantry such as and. The explosion left by an HEC round also serves as a one-way tracer, revealing the location of shot placement while preventing the enemy from leading any projectile path back to the source. The HEC round tends to be only effective in specific applications, warranting its relatively limited use in the UNSC Defense Force. An HEC round is labeled with a white-over-red-painted tip.

8.3×38mm SLAP
The 8.3×38mm SLAP is based on the 11.5×34mm necked down to accept a 8.3mm sabot encasing a 5mm Staballoy penetrator. The sabot allows for a smaller, lighter projectile in a relatively large cartridge, resulting in a significantly higher velocity and sectional density when fired. The dense, high-velocity projectile offers very effective armor penetration capabilities, exceeding those of the and even the. Also due to its high velocity and light weight, the 8.3×38mm SLAP possesses a much flatter trajectory when fired, resulting in a greater effective range and better accuracy compared to those of more-standard rounds. Furthermore, Staballoy is pyrophoric, combusting easily at high temperatures. As a result, the 8.3×38mm SLAP is effective at destroying lightly-armored vehicles such as and ; once it pierces plating, it can ignite fuel with the possibility of detonating the entire vehicle. As the cartridge possesses the same overall dimensions as those of the 11.5×34mm, an M6S115 only needs to replace its integral suppressor-muzzle-brake system with one that would accept the 8.3×38mm would be a simple barrel swap.

8.3×38mm Subsonic AP
The 8.3×38mm Subsonic AP is intended specifically to be used in conjunction with suppressors in order to generate as little sound as possible when fired; as such, it is ideal for the integrally-suppressed M6S115. With the projectile traveling at less than the speed of sound, no sonic "crack" is produced. In order to compensate for the substantially lower velocity, the subsonic projectile is larger and heavier than that of the SLAP cartridge, and instead consists of a dense tungsten core jacketed in phosphor bronze with no sabot. No amount of mass, however, can fully make up for the substantially lower velocity the Subsonic AP round has when compared to that of the SLAP round; the Subsonic AP cartridge, therefore, possesses inferior armor penetration and wounding capabilities than those of the SLAP. Like the 8.3×38mm SLAP, the 8.3×38mm Subsonic AP possesses the same overall dimensions as those of the 11.5×34mm, so the only adjustment to an M6S115 that would need to be made in order to accept the cartridge would be a suppressor-muzzle-brake-assembly replacement.

Trivia

 * The artwork from which the M6K is derived is by Leo Fox.
 * I consider the  analogous to the present-day .45 ACP, the former of which is a large, standard-issue handgun cartridge, and the latter of which is a relatively large handgun cartridge that is standard-issue in many modern militaries.  I decided to make up a handgun cartridge for the UNSC that would be analogous to the smaller, present-day 9×19mm Parabellum.  I settled on basing the dimensions of the cartridge off those of the real-world .454 Casull, as the 12.7×40mm’s dimensions are very similar to those the real-world .500 S&W Magnum.  In terms of dimensions, the 11.5×34mm differs from the .454 Casull in that its case is 34mm long as opposed to being 35mm in length, very much like how the 12.7×40mm’s case is 1 mm shorter than the .500 S&W Magnum’s.  Another difference between the M6I's 11.5×34mm and the .454 Casull is that the former is rimless while the latter is rimmed.
 * Due to the limited canonical information in regards to the round used in the, I have decided to create my own details for the ammunition type and used Lehigh Defense's Xtreme Penetrator ammunition for inspiration.
 * The properties of the 8.3×38mm SLAP cartridge is based on those of the 6.5×25mm CBJ Ball cartridge, which can allegedly pierce armor more effectively than the 7.62×51mm NATO (likely M80 FMJ) can.