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(RP) Firearms of the Old Federation (MX-4 Platform Standard Infantry Weapon)

The Unified Terran Federation, now known as the Old Federation, possessed perhaps the largest unified military in human history; myriad branches and services structured together to fulfil the requirements of humanity's defence interests in the stars. This, of course, included a unified, standardised firearm series with 'pattern's blueprinted for mass production across the dozens of solar systems controlled by the Old Federation at the height of its power in the early 41st century A.D.

These firearm designs were produced in such vast numbers that they can still be found by the 45th century, even in factory-new condition, despite the complete collapse of the Old Federation and reduction of many of its worlds to ruins. The reliability and standardisation of the platforms developed by the UTN is reflected in the fact that almost all of the base designs are still in service with the current Aquarian military power, ATIS, some few hundred cycles later, with only a few major changes to each design.

MX-4 Platform Standard Infantry Weapon


The MX-4 is widely recognised as the defacto standard of the Old Federation's foot soldiers. It was developed in the early 2800s as a replacement to the MX768 Intermediatory Battle Rifle Platform due to advancements in integrated, compact FIMD technologies allowing higher chamber pressures and velocities with significantly smaller munitions delivering the same or greater damage to the target.

The MX-4 has its origins in a project by the Western Alliance to replace the original, ancient M4 carbine weapon using the equally ancient old-earth 5.56mm cartridge in the 20th and 21st centuries. The MX-4 platform was originally proposed by the TSRC for testing advanced, then-prototype FIMD technology some time before the end of the Fossil Wars, but was rejected by the Western Alliance who had already adopted an intermediate 6mm cartridge based on CIMD technology.

The design was archived and later rediscovered by the Unified Terran Nations after the TSRC made contact with the Federation in 2415, 5 years after the founding on Eridonia. The TSRC had worked with the fledgling federation to fast-track advanced designs (many of which had been prototyped decades before the Colony Ship Eridonia left Earth). FIMD technology was one of such advances - with the TSRC assisting Federation engineers to create the first series of 7.68MM Fission Impulse Mass Driver projectiles, and with it, the first-generation MX768 battle rifles.

The MX768 would remain in service with the Federal Military until the early 2800s where Federal Weapons Design Bureau requested a smaller, more accurate yet higher lethality armour-piercing cartridge based on newer, Fusion Impulse Mass Driver technology. The TSRC utilised the base design for the original MX-4 rifle and modernised it, presenting it to the FWDB in the mid 2800s. The Design was approved and entered service in the middle of the century, eventually phasing out the pattern B MX768 rifles, which were relegated to Planetary Guard Stocks entirely by 3000 A.D.

It is interesting to note that several times in human historical firearms design, smaller cartridges replaced larger ones, even when larger ones had began to replace smaller designs by the early 22nd century - ultimately advances in FIMD and penetrator technology allowed smaller rounds to achieve 'sufficient' lethality against human targets without needing to be physically large (hence the 4mm cartridge).


The MX-4 is fairly bulky weapon, despite using the relatively small FIMD 4mm cartridge. This is innate to essentially all FIMD weapons technology due to the considerable active cooling systems required to regulate the temperature of the firing assembly and barrel construction during operation. The distinctive 'blocky' design is signature of this technology, as it contains the heat-sink cells and fluid reservoirs required to facilitate this cooling system. This cooling system uses highly pressurised coolant fluid (typically liquid helium or liquid hydrogen) to exchange thermal energy, which then vaporises and is exhausted from the heatsink assembly. The highly compressed coolant required for firing a single round is entirely contained within the cartridge.

Despite the size, the weapon is not overly heavy. The design incorporates advanced (for the time) macropolymer construction with low-metallic composition, with low mass, high tensile strength characteristics. This macropolymer furniture and frame is also extremely durable - being able to take significant abuse before braking, contributing to the reputation for rugged reliability that these weapons earned in service, and indeed, after the Fall of the UTN.

The larger size is also due to a design emphasis with the original MX-4 platform- intended for highly accurate, high volume of fire with a high velocity, but small, projectile. This allows the MX-4 to function in a SSW (Squad Support Weapon) in a suppressing fire role with volume of fire, similar to how an LMG would be operated in earlier armies. The MX-4 platform had several magazine types developed, all of which could be interchanged with the weapon system, in the field, with no re-tooling required. One of such designs was a large, 100+ round box magazine allowing the MX-4 to lay down impressive suppressing fire in an SSW role.

The MX-4 platform weapon features an integrated display and digital micro-controller that handles basic data processing functions such as diagnostics, thermal regulation, ammo count, and basic targeting data. Later models featured a full-spectrum optical targeting system with magnification and image enhancement (such as Thermal or Low Light optics) on the integrated display.

The Standard Magazine holds 59 rounds in a dual-feed straight box design, which is located behind the hand grip - giving the MX-4 the distinctive 'bullpup' layout. The 59-round magazine is fairly large for a 4mm cartridge design (smaller, more compact magazines were available for scout weapon variants) but the FWDB placed emphasis on soldiers being able to fire for longer without having to reload. With training encouraging small, but highly accurate 3-5 round bursts- such weapons could overwhelm enemy infantry fairly quickly, especially when fired in full squad co-ordination.

The original OF Pattern-1 MX-4 weapons had a forward hand-guard located under the muzzle section, designed to protect the operator's hand when using the weapon against impromptu cover or makeshift barriers. In addition, this guard could also mount a folding bipod for when the weapon was used in SSW capacity; allowing effective emplacements to be established with little effort. Later variants lacked the hand-guard to save volume and mass, and cost of production, however.

All patterns of the MX-4 platform feature a fixed, non-collapsible frame-stock for shoulder-fire, despite the extremely low recoil of the weapon. The weapon features an interchangeable top-mounted attachment rail for various additional scopes, sights and modules. For basic operation, the weapon incorporates illuminated iron sights as a backup in case of a failure of the integral sighting system.


The MX-4 is known to be an extremely easy-to-control weapon; with very light recoil and muzzle climb in large part due to the extensive internal recoil-compensation mechanisms. The entire firing assembly is suspended in a Pneumatic Pressure suspension system allowing the firing assembly to reciprocate in addition to the bolt carrier when cycling a round. The barrel is telescopically sleeved with a fluid seal on the muzzle end to allow this without barrel movement. Combined with magnetic stabilisation on later patterns, the MX-4 is extremely easy to control, has low recoil and impressively accurate even at range.

Due to the nature of FIMD technology, the MX-4, like all FIMD-firearms, has innate thermal concerns - somewhat offset by the extensive cooling system, but not fully nullified. Barrel warping is one of the most significant of these concerns, due to the extreme (nuclear) energies released by each Catalyst Ignition, without active Fluid Cooling (typically Liquid Helium) thermal regulation, the barrel would disintegrated after only a few shots.

The cooling system produces a significant amount of helium vapour after each shot, which can build up leading to visible exhaust sprays from the heatsink assembly. In extreme cases, prolonged firing of the weapon can visibly impede the shooter's vision. Typically, the weapon is not operated for long periods of time, but in SSW roles - this was a fairly common problem among the earlier patterns of the weapon. Later variants had improved exhaust outlets that directed the sprays downward, but only alleviated the problem, rather than solving it entirely.

In addition to vapour expulsion, thermal cycling of the barrel and firing assembly was a major concern with pattern-1 weapons due to the cast Tritanium (Macropolymer-infused Titanium alloys) chamber developing cracks after extensive operation. Later variants solved this issue with fully macro-polymer internals when such heat-resistant molecular compositions became available during the UTN's rapid industrial advancement in the early 30st century.

All variants of the MX-4 platform feature computer-controlled thermal regulation with internal sensors on the firing assembly and barrel construction- warning the operator via the integral display of excessive thermal build-up.


The MX-4 platform weapon utilises the FIMD 4mm cartridge from which it derives its name in the UTN Standard weapon naming system. The FIMD 4mm cartridge is a small, high velocity dual-purpose, smart-configurable round with the ability to be dynamically configured by the weapon system in the field, as per operator requirements.

The basic design of the FIMD 4mm round includes a sub-calibre rod-penetrator constructed of Tungsten, with a rear fin-stabilisation array. The rod-penetrator is enveloped in a softer, expanding 'jacket' which also doubles as the sabot for the penetrator. In Armour Piercing mode, the jacket is discarded after the projectile leaves the barrel, releasing the rod-penetrator at extreme velocities. Due to the design, the penetrator is the full length of the cartridge itself (longer than the jacketed 'bullet') and has extremely high pure armour penetration characteristics.

Effective penetration of the rod-penetrator when the round is configured in AP mode is around 50-60mm of RHA steel. This was primarily developed to defeat enemy infantry equipped with heavy power armour, or armoured exo-skeletons.

Disassembled FIMD 4mm projectile. Note the long subi-calibre projectile with fin stabilisation, and the full-calbre expansion/discarding sabot 'jacket'

In Soft Target mode, the projectile retains the expanding jacket around the tungsten core as a complete, full calibre projectile. Upon impact, the jacket expands, delivering high amounts of kinetic energy to the target, while the penetrator is released, entering the target and fragmenting in a manner consistent with ACPR rounds of old. Though the armour penetration is still effective at 20-25mm of RHA steel, the significantly lower velocity of the projectile means that it is still shy of the penetration of the penetrator in sabot operation.

The weapon system can be selectively toggled, per-round, by the operator via the integrated display, or on later models, via the integration to the soldier's suit interface, or automatically based on BAS-enhanced X-ray target compositional analysis (Pattern 3 and IMX-4 models in service with ATIS).

In standard configuration, the MX-4 firing the FIMD 4MM projectile in Armour piercing mode achieves a muzzle-velocity in 1G, ~1020mb atmospheric pressure environment of around 2200 m/s, while in Soft Target mode, the velocity is around 1400m/s.

Like all FIMD cartridges that saw active use, the FIMD 4MM is fully cased, largely due to the integrity requirements for withstanding Catalyst Ignition pressures innate to the technology. Caseless variants were trailed but were largely unsuccessful due to premature cook-offs (an attribute of caseless cartridges well known throughout their trials in history, further exacerbated by the extreme energy/pressure tolerances required of FIMD casings).

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