(Ammo)(UTN) FIMD/DP 4MM

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.

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).