M84 heavy machine gun

The M84 heavy machine gun was a recoil-operated, belt-fed, fully automatic heavy machine gun in service with the UNSC Army and Marine Corps from 2529 onwards. Chambered for the 12.7x99mm (.50 BMG) cartridge, the M84 provided UNSC forces with superior range, accuracy and terminal effects compared to machine guns and automatic rifles chambered in smaller calibres, at the expense of increased weight and reduced portability. This increase in mass meant that only personnel using powered armour or exoskeletons could fire the weapon without a mount of some sort; combined with its high damage characteristics, this resulted in the M84's extensive use mounted to vehicles and aircraft. The machine gun was produced in two variants; the M84 A1 had a conventional layout and an integral heavy bipod, for use by infantry; and the A2 which replaced these with a universal mount and spade grip, for use from tripods, aircraft and vehicle mounts. The M84 was effective against infantry, unarmored or lightly armored vehicles, light fortifications, and low-flying aircraft.
==History==
The M84 was an evolution of the , which was the UNSC's mainstream heavy machine gun during the insurrction, and the first four years of the Human-Covenant War. Although widely used and highly effective, the M247 was limited to vehicle and aircraft-mounted use, and limited use by SPARTANs, because of its prohibitive weight and bulk. This meant that the vast majority of infantry units within the UNSC went without an infantry deployable heavy machine gun which, considering its sheer power, would have been an imperative weapon in combat against physically superior species such as Jiralhanae and Sangheili. The M84 was essentially a complete update of the M247H design, aiming to replicate and improve on its basic effectiveness while expanding its use to both vehicles/aircraft and infantry, as a true heavy machine gun. This led to the development of two similar but separate models; the M84A1, for use by mobile infantry, and the M84A2, for stationary use by infantry and mounting to vehicles and aircraft.
Misriah Armouries began redesigning the latest iteration of the weapon, updating the M247H with modern materials, technology and design innovations. Beginning in 2527, the weapon had completed the testing phase by 2529, and was in full production the year after. The M84 was used extensively throughout the Great War in both infantry and vehicle-mounted roles, where it proved effective at defeating the personal shielding and armour of Covenant heavy infantry. Due to both the weapon's sheer power and its ubiquity throughout UNSC armed forces, appearing alongside vehicles, infantry and aircraft in large numbers, the M84 was one of the most heavily relied upon support weapons in defeating Covenant infantry; more powerful weapons were generally less available and more common ones lacked the firepower to have such an impact. In particular the M84 was used against the 'swarming' mass tactics used by Unggoy, as its rate of fire and ballistic characteristics made it superb at cutting down and cutting through advancing infantry. The M84 was famed for its ability to protect vehicle convoys during ambushes by numerically superior enemies.
==Usage==
The M84 heavy machine gun filled a wide range of roles; primarily for providing heavy fire support for vehicles or infantry from a variety of mounts, and also acting in an anti-materiél role. The M84 was one of the most widely used support weapons in the UNSC's arsenal. In Marine Corps and Army infantry units, the M84 was issued as a ground-portable, crew-served machine gun with heavy weapons platoons to provide fire support to rifle and weapons companies. M84 operators within a platoon would provide sustained automatic fire on enemy positions and targets, filling a support and direct fire role and augmenting the volume of fire an infantry unit could put downrange. In addition, it was capable of providing sustained suppressive fire against enemy targets, affecting the accuracy of their return fire and making it easier for other personnel to close with and eliminate the enemy. In this way the M84 complemented the lighter and more numerous M62 light machine gun and M73 machine guns in the sustained fire role. The M84 was operated by a crew of two, with one firing and one feeding ammunition and spotting targets in combat, and carrying the weapon and its ammunition respectively when not in action.
The M84 was useful at nearly all ranges, at close range its high rate of fire making up for its large size and at longer ranges its large calibre round maintaining accuracy and kinetic energy. Strong individuals such as SPARTANs were fully able to utilise the weapon without a bipod or tripod, but for the average operator such aids were necessary to control the weapon's recoil and ensure accuracy at longer ranges. Use of the M84 from a prone position with a bipod increased the probability of a hit considerably and made best use of the long range offered by the .50 BMG calibre. The weapon's maximum range was seven kilometres and its maximum effective range was just under two kilometres, when mounted on a tripod. At under this range its rate of fire and large round made it a serious threat to infantry, light vehicles and low-flying aircraft.
As well as usage by infantry, the M84 also saw use with aircraft and armoured vehicles, where it provided a versatile anti-personnel and anti-light vehicle weapon. The machine gun was often mounted to armoured fighting vehicles, where it provided a defensive capability against a range of threats. In this configuration the M84 could be manually operated through the vehicle's roof hatch by the operator, or electronically operated remotely from inside the vehicle, using the overhead weapon system (OWS). The M84 was also employed by aircraft such as light troop carriers and dropships, where it was used to offer suppressive and protective fire to troops while they disembarked or boarded the craft.
The M84 was effective against virtually all infantry types fielded by the Covenant, against which it was used to great effect during the Great War. Weaker forms such as Unggoy and Kig-Yar offered little resistance, and larger forms such as shielded and armoured Jiralhanae quickly succumbed to the weapon's high velocity, large calibre ammunition and its fully automatic nature. The weapon was less effective against the armour of Hunters, however could realistically defeat them with sustained fire. Later, when UNSC forces combated Prometheans, the M84 and the 12.7x99mm cartridge were found to be effective against their exoskeletal armour. Not just effective against infantry, the M84's larger calibre made it sufficient for engaging lightly armoured vehicles and some low-flying aircraft. The round could be relied on to penetrate most light armour and damage internal equipment and personnel repeatedly with its sustained automatic fire capability.
==Design==
The M84, unlike most of the UNSC's infantry weapons, made use of a conventional rather than a bullpup layout, in order to more evenly distribute the weapon's considerable weight. Although this reduced barrel length in comparison to the weapon's length, it still maintained a respectable barrel length of thirty two inches. The weapon was fed caseless 12.7x99mm rounds by a metal disintegrating link belt usually held inside a rigid belt container mounted on the side of the weapon; these containers were removed and replaced once the belt was expended and came in 100, 200, 500 and 1000 round sizes. The M84's rail interface system consisted of a total of three attachment rails; one rail along the weapon's upper receiver, and one each on the weapon's sides. The weapon was equipped with fixed day and night iron sights which could be adjusted for windage and elevation. A heavy duty integral bipod was mounted underneath the weapon's gas block, which folded down to provide a lightweight yet sturdy firing base. Although the weapon's stock itself was fixed, the butt plate could be extended rearwards if necessary by the operator. The stock also featured an adjustable cheek rest for more comfortable sustained fire. The fire mode selector was located directly above the ergonomic rubberised pistol grip, and featured safe and fully automatic fire modes only. The weapon's side furniture, stock and grip were constructed from lightweight fibre-reinforced polymer, made of a polymer matrix reinforced with fibres (a carbon fibre/kevlar mix). This material allowed for a lightweight though immensely strong construction. The upper and lower receivers and rails were constructed from high grade corrosion resistant aluminium, while the firing mechanism was reinforced precision-machined titanium alloy. The weapon featured no quick barrel replacement mechanism, meaning the weapon needed to be stripped to replace the barrel. However, the fluted barrel was constructed from a titanium/ceramic metal matrix composite lined with a super heat and friction resistant silicon carbide ceramic matrix composite. This gave the barrel an effective life of 60,000 rounds before it needed replacing, as well as reducing its weight over steel barrels by fifty percent, and largely eliminating overheating and accuracy problems even after sustained fire. The microprocessor kept count of rounds fired since the barrel's replacement and gave alerts before the barrel needed replacing. The weapon was modular, meaning that it was assembled in interchangeable sections that were easy to separate and replace, in addition to its rails which accepted a wide range of attachments. 
The M84 was a light weapon for its size as a result of its weight saving materials, however its size still gave it substantial weight compared to weapons such as light machine guns. This, coupled with its larger calibre and fully automatic role, meant that recoil control was imperative. Several hydraulic buffers linked to the weapon's microprocessor controlled backwards recoil by attempting to cancel it out, and reducing it considerably. The weapon also featured a venting system that recovered part of the gases generated by the round, and pushed it back in a space located behind the bolt during the cycle. This caused the bolt to 'bounce' on a sort of 'gas cushion' that acted as a buffer, dramatically reducing the weapon's recoil. It also featured energy absorption mechanisms in the buttstock, absorbing energy and reducing felt recoil. The barrel end featured a large flash suppressor which reduced the barrel's muzzle flash and directed it away from the shooter's vision; as a secondary function this vented excess gas in a way counteractive to the weapon's muzzle climb and backwards recoil.
The M84's charging handle was located to the rear of the fire mode selector, and was accessible through both sides of the weapon. As the M84 utilised caseless ammunition, it lacked an ejection port or any mechanism of extracting and ejecting a casing.
==Ammunition==
The M84 heavy machine gun used the 12.7x99mm (also known as .50 BMG) heavy machine gun round, which gave it superior range, accuracy and terminal effects over intermediate and full sized cartridges.
The weapon, like nearly all UNSC weapons, utilised caseless ammunition, which had several advantages over traditional cased ammunition. This was a major change from Great War-era weapons, which utilised rounds which contained propellant inside metal casings. From the conclusion of the Human-Covenant War onwards, the UNSC worked on increasing the lethality of its infantry firearms; aside from the development of directed energy weapons, one element the UNSC looked at was the application of caseless ammunition. This was already used with operational success in the M7/Caseless Submachine Gun, though was limited to this weapon and not available in other calibres, not capitalising on the weapon's success and complicating logistical operations. Developments in technology, however, allowed the widespread adoption of caseless ammunition from around two decades after the war onwards, with newer replacement weapons discarding more traditional ammunition.
Unlike experimental caseless munitions of the late 20th century, the M84's ammunition maintained a tolerance similar to traditional, cased bullets. This was achieved by the propellant being coated in a thin layer of water, heat and dirt resistant coating. This also allowed rounds to be handled and manually loaded, as well as improving their 'drop resistance' or shock tolerance significantly. This also increased its shelf life considerably. These qualities were present in the M7/C Submachine Gun's ammunition, and carried over to the UNSC's next generation of caseless weapons.
Caseless ammunition offered several advantages. The most noticeable was an increase in muzzle velocity; not only was more propellant available per bullet with the caseless system, the propellant released significantly more energy when combusted. This higher muzzle velocity led the M84 to deal noticeably increased damage to the target in terms of kinetic effects; this also markedly improved its armour and shield penetration characteristics. The 12.7mm round's muzzle velocity was typically two to three times higher with caseless ammunition than traditional cased types, making it significantly more effective than its more traditional predecessors and meaning even assault rifles were capable of taking on shielded and armoured Covenant infantry. It was largely this increase in lethality that convinced the UNSC to maintain ballistic weapons for the bulk of their weapons, rather than developing and adopting plasma weapons of their own.
Caseless ammunition also reduced the complexity of the M84's internal mechanisms, reducing weight and disassembly time, as there was no need for extraction and ejection of spent casings. As the overall round was smaller, it allowed for larger capacity magazines, and its significantly reduced weight increased the amount of ammunition a soldier could realistically carry, which was especially significant for a heavy machine gun.
The bullet was actually embedded inside the solid propellant 'block'; this, the bullet and the primer were held together by a combustible glue. When fired, everything in the chamber save the bullet fully combusted, leaving no residue in the chamber and propelling the bullet down the barrel. The propellant itself was an advanced solid, plasma-based substance similar in composition to that of the the experimental M634 HP-SAP, which gave high muzzle velocity; this in turn increased lethality on contact with the target. The block was highly resistant to temperature, which prevented it from combusting prematurely, for example on contact with heat inside the weapon, or external heat sources.
There were numerous forms of .50 BMG ammunition available to the UNSC, each differing in usage and effects, which, alonside impressive ballistic performance, was a prime factor in the cartridge's longevity. These ammunition types varied from the simple and cheap to the complex and expensive to manufacture. The most commonly issued and utilised round was the M697 S/AP-HE, or Shield/Armour Piercing-High Explosive, which was a balance of potency and cost, although more expensive M696 SAP-HEI was highly effective in the anti-materiél role.
*M696 Semi-Armour Piercing-High Explosive Incendiary: The M696 Semi-Armour Piercing-High Explosive Incendiary, or SAP-HEI, was an ammunition type combining both an armour piercing and an explosive/incendiary capability. The bullet's tip was filled with a highly incendiary chemical, which burned at several thousand degrees upon impact with a target, damaging or melting armour and heavily affecting shields. Behind this was a high explosive component which detonated immediately on impact with a target, further damaging the area. Behind this was a solid core penetrator of depleted uranium, held in a backing 'cup' of steel. The components were held together in a copper or lead jacket. Upon hitting a target, the incendiary and explosive properties would damage the target area, aiding considerably in the depleted uranium's penetration into the interior. The penetrator featured a self-sharpening tip and was itself pyrophoric, meaning that it ignited upon impact with the incendiary material. It then punched through any remaining armour, having been ignited at this point and also carrying with it any remaining incendiary material from the tip. Effects on the target, especially if it was organically-based, were catastrophically damaging; the round would typically bypass any present armour (following rapid depletion of shields) and propel an incendiary penetrator and secondary incendiary material into the target's innards.
*M697 Shield/Armour Piercing-High Explosive: The M697 Shield/Armour Piercing-High Explosive, also known as S/AP-HE, was the most common and effective type of ammunition employed in the .50 BMG calibre. The bullet itself was coated in a polymer which acquired an electric charge while in flight, aiding the bullet's kinetic energy in depleting shielding and, although it had little effect on armour, increased the bullet's drain on shielding by roughly half. The further the bullet travelled in the air, the greater the charge it acquired, so this effect was at its most potent at longer ranges and nearly negligible at extreme close range. The tip of the round consisted of a deforming ballistic cap; a lightweight element which crumpled upon impact with a target and gave the bullet superior aerodynamic characteristics. The bullet's armour-piercing core was a 'CVT' (Chromium Vanadium Tungsten) and Austenitic Steel alloy with a self-sharpening tip; when it fractured upon impact, it would do so in a way that the remaining element was still a sharp point. Behind this was a pre-fragmented block of 'TC3' alloy, composed of tungsten, cerium and copper carbide, with a delayed action fuze in the centre. This alloy maintained similar incendiary and pyrophoric properties to uranium, though without associated radiological effects. A millisecond after the bullet's penetration of the target, the fuze would activate; this fuze contained a plasma-based high explosive compound, which explosively fragmented the pre-weakened TC3, in turn heavily damaging organic structures and internal organs. As a secondary function the TC3 was incendiary, causing severe secondary damage to soft targets.
*M698 Jacketed Hollow Point: M698 Jacketed Hollow Point (JHP) round was a specialised munition for unarmoured and lightly armoured infantry, being at its most effective when utilised against Kig-Yar and Unggoy.
*M699 Tracer: The M699 Tracer
*M700 Armour Piercing High Explosive: The M700 Armour Piercing High Explosive round (AP-HE) was an effective armour defeating round, used as a cheaper alternative to more expensive semi-armour piercing high explosive incendiary (SAPHEI) ammunition. The bullet's tip was composed of a hollow, lightweight ballistic cap which deformed on impact, and improved the bullet's ballistic properties. Behind this was an armour piercing penetrator composed of CVT (Chromium Vanadium Tungsten) and Austenitic steel alloy, with a self sharpening tip. The remaining length of the bullet was comprised of 'stressed' steel; the interior of this steel was hollowed out and contained explosive filler and a delayed action fuze. Shortly after impact this would detonate, fragmenting the steel and causing effects similar to a miniature fragmentation grenade, albeit designed to detonate inside an organic body.
*M701 High Explosive Squash Head: The M701 High Explosive Squash Head round, or HESH, was designed to deal damage to a target without needing to defeat its armour, thus making it ideal for shielded and armoured targets. The bullet was formed of a thin steel shell filled with plastic explosive, with a delayed action base fuze towards the rear. Upon impact with a target the bullet would deform and form a disc or 'pat' of explosive with an increased surface area. A millisecond later the base fuze detonated, creating a shock wave that, owing to its large surface area and direct contact with the target, was transmitted through the material. The round was able to effectively defeat active shields due to primary kinetic, secondary explosive and tertiary kinetic effects. If impacting on armour, the round would cause little damage to the armour itself but directly damage the target through the resulting shockwave. This shockwave was highly disruptive to internal organs, often tearing vital organs from their connective vessels or reducing them to a thick paste. In some cases, the shockwave would cause a secondary effect known as spalling, where minute fragments of the armour's internal layer would be projected off it at high velocity, known as spall. In these cases secondary damage was enhanced by a sort of fragmentation effect inside the target's own body, and caused by their own armour. The round was relatively cheap to produce and was effective against unarmoured, armoured, unshielded or shielded infantry targets.
==Variants==
The M84 was produced in two main variants, which was necessary due to the wide range of uses the weapon found itself in.
===M84A1===
The M84A1 was laid out like a standard machine gun, with a fixed stock and pistol grip. Due to the A1's light weight of 19.5 kilograms, as well as its recoil reduction mechanisms, the weapon was more suitable for such use than earlier heavy machine guns. However, its recoil was still far too high to be fired from the shoulder or the hip, so the design included an integral heavy bipod which folded down for firing. This bipod was not removable and was strong and stable enough to allow accurate fire. The A1 variant was issued to infantry units, who used it as a bipod-fired weapon, and SPARTANs, who were strong enough to wield the weapon like a rifle. The A1 and its tripod together weighted 19.5 kilograms, compared to the 29 kilograms of a more bulky tripod-equipped M84A2. As a result of this, most infantry favoured the M84A1 for its light weight and mobility, which was an advantage in close quarters, highly mobile environments such as urban or forested areas. This high mobility as meant the machine gun could be moved almost as fast as the infantry it was supporting, and was able to fire from almost any location.
===M84A2===
The M84A2 saw its fixed stock and pistol grip replaced by a spade grip, and a universal weapon mount added to the weapon's underside. This allowed the weapon to be mounted onto aircraft and vehicle weapon mounts, as well as deployed by infantry using a tripod. Although the tripod provided a more stable firing platform than the A1's bipod could, this came at the expense of size and weight, being much more bulky and weighing close to 30 kilograms rather than the A1's 19.5 kilograms. The M84A2's usage was mostly limited to vehicles and aircraft as a result, although tripod-mounted A2s were still a common sight within infantry units. The A2 could also be electronically fired, allowing it to be mounted to the exterior of a vehicle such as a tank and operated remotely from within.
==Gallery==
Image:M84A2_HMG_mod1.png|The M84A2 mounted on a lightweight tripod.
Category:UNSC Weapons