11.5×34mm

The 11.5×34mm is a large handgun cartridge for use in some of the.

Cartridge dimensions
The 11.5×34mm consists of an 11.5-millimeter-diameter bullet in a 34-millimeter-long case. The case of the cartridge is rimless.

Performance
As a magnum cartridge, the 11.5×34mm has more than enough power to stop a human-sized target in one shot. While the 11.5×34mm is less powerful than the larger cartridge, it possesses less recoil and its smaller size enables greater magazine capacities in firearms that are chambered for it.

M330 SAP HE
The M330 (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. The M330 is the standard-issue 11.5×34mm cartridge of the  and is labeled with a yellow painted tip.

M335 SAP HP
The M335 (SAP HP) projectile is made entirely of phosphor bronze 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. Compared to the M330, the M335 sacrifices some potential to deal internal damage in order to improve its ability to penetrate armor and other barriers. The M335 is the second-most common 11.5×34mm cartridge used by the UNSC.

M423 HEC
The M423 high-explosive concussive (HEC) round possesses a copper jacket with more explosive filling than that of the M330, which is detonated by an incendiary material at the tip that is ignited by the round's impact. Instead of penetrating the surface and then exploding inside the target, an M423 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 lower recoil of the 11.5×34mm cartridge, multiple consecutive M423 rounds are capable of pinning down heavier infantry such as  and. The explosion left by an M423 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. Inevitably, the HEC round is less lethal than the SAP HE round and is only effective in specific applications, warranting its relatively limited use in the UNSC Defense Force. An M423 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, the only adjustment to a firearm that would need to be made in order to 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. 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 a firearm that would need to be made in order to accept the cartridge would be a barrel change.

Civilian
Various ammunition manufacturers make other variants of the 11.5×34mm cartridge for the civilian or market, such as full metal jacket, hollow-point, and polymer-tipped, the first of which is typically used for target practice, and the latter two of which is mostly used for defense.

Trivia

 * 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.