M73 machine gun

The M73 machine gun was a gas-operated, belt-fed general purpose machine gun used by the United Nations Space Command during the Human-Covenant War. As a general purpose or medium machine gun, the M73 bridged the gap between light and heavy machine guns, and found a wide range of uses within the branches of the UNSC, including but not limited to use by infantry, aircraft and ground vehicles. Chambered for the 9.5x60mm high velocity, full-sized rifle cartridge, the M73 supplemented weapons using lighter cartridges in squad-based combat, providing increased range and terminal effects compared to smaller calibre small arms and supporting infantry units with sustained heavy fire. The M73's weight made it significantly more unwieldy than assault rifles and light machine guns, although a single soldier could still carry it effectively into battle; however, its large calibre and sustained fire role meant that accurate and controlled fire necessitated use of a bipod or tripod.
==History==
Soon after the introduction of the , it became apparent that the weapon's 7.62x51mm NATO cartridge, used in assault rifles and light machine guns, was too light for effective use in a medium machine gun. Although it offered superior sustained fire abilities compared to other weapons of the calibre, it was recognised that a replacement with a more powerful cartridge would be needed to fill the role of medium machine gun. Notably, the M247 was too large and heavy to be used by standard infantry in combat, which was unusual due to its small calibre and resulted from its initial design as a vehicle-mounted weapon. In 2531 the UNSC tasked Misriah Armoury with producing a smaller, lighter weapon capable of sustained automatic fire chambered in the 9.5x40mm KURZ experimental cartridge; rather than design an entirely new weapon, they turned to the existing and modified it. The resulting weapon was the BR55HB Light Support Weapon, which was designed with a longer barrel and integral folding bipod. The BR55's shallow magazine coupled with mechanical unreliability in fully automatic mode made it unpopular with infantry automatic gunners; following its poor performance, the weapon was issued as an 'interim solution' to the problem while Misriah worked on an entirely new design called the XM73. The BR55HB then underwent an improvement program, which removed its bipod and strengthened its internal parts, allowing it to be widely issued as a successful battle rifle.
The XM73 e was initially designed for the conventional 9.5x40mm 'Short' cartridge, but during its design period was altered to fire the caseless 9.5x60mm cartridge. Caseless ammunition of this type eventually equipped all UNSC small arms. The finished product of extensive designing and testing by Misriah was the M73 machine gun, which was introduced in late 2543. Operated by both the UNSC Marine Corps and the UNSC Army, the M73 proved to be a reliable and effective weapon which, when used in a stationary position, had the rate of fire, range and terminal effects to significantly increase the lethality of infantry units in combat against Covenant forces. The UNSC ultimately procured over 4,000,000 units, seeing extensive use throughout the remainder of the 26th century.
==Usage==
As a general purpose machine gun, the M73 filled a wide range of roles, primarily for providing fire support for vehicles or infantry from a variety of mounts. Infantry formations were issued the M73 at platoon level to complement the lighter M62 light machine gun, which was issued at the squad level. The M73 was designed to be operated by 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. However, the weapon and its new, far lighter caseless ammunition, proved light enough for a single trooper to carry effectively and operate in combat; the use of drum magazines more than belts also for the most part negated the need for an extra operator to feed the ammunition.
M73 operators within a platoon would provide sustained automatic fire on enemy positions, 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. The M73 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 M73 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 9.5x60mm calibre. The weapon's most effective range, however, was between fifty and eight hundred metres, where its large magazines and fully automatic ability made it a serious threat and its relatively unwieldy nature was not an issue. Accuracy at longer ranges was possible with the 9.5mm calibre and achieved through the use of optics such as telescopic sights. The M73 was an effective weapon in most combat environments, though as a rule fared better in battlefields where close quarters contact with the enemy was rare.
As well as usage by infantry, the M73 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 M73 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 M73 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 M73 was effective against virtually all infantry types fielded by the Covenant. 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. Not just effective against infantry, the M73'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 M73, unlike most of the UNSC's infantry weapons, made use of a conventional rather than bullpup layout, in order to more evenly distribute the weapon's weight. Although this reduced barrel length in comparision to the weapon's length, it still maintained a barrel length of 27.5 inches. The weapon was fed caseless 12.7x99mm rounds by a 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 M73's removable rail interface system consisted of a total of four attachment rails; one along the weapon's upper receiver, doubling as a carrying handle, one along the bottom of the forward handgrip and two on the weapon's sides. The weapon was equipped with day and night iron sights which could be adjusted for windage and elevation; these sights were not removable but could be folded down when not in use. Side-mounted rails would commonly be used for laser aiming modules, rangefinders or flashlights, or a range of more advanced aids for the operator including, for example, folding LCD displays which could be linked to the user's HUD and electronic sight. An integral bipod was mounted underneath the weapon's gas block, which folded down and extended to provide a lightweight yet sturdy firing base. This could be removed and replaced with a heavier tripod for less mobile sustained fire roles, although it made the weapon heavier and more cumbersome to move. 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, four round burst and fully automatic fire modes. Directly forward of the belt container mount was the low profile release catch, which was located along the weapons mid line allowing access for both left and right handed shooters. 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). The material allowed for a lightweight though immensely strong construction. The upper and lower receivers and rails were constructed from high grade steel, while the firing mechanism were 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 30,000 rounds before it needed replacing, as well as reducing its weight over steel barrels by 56%, and largely eliminating overheating weapons even after sustained fire. 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 M73 was a relatively light weapon for its size as a result of its weight saving materials, however still weighed much more than weapons such as assault rifles, battle rifles and 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 weapon's bullpup nature meant that it was somewhat back-heavy; however, the recoil reduction mechanisms largely balanced this out, and its long and heavy barrel and forward grip combated any muzzle climb. 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 M73's charging handle was located to the rear of the fire mode selector, and was accessible through both sides of the weapon. As the M73 utilised caseless ammunition, it lacked an ejection port or any mechanism of extracting and ejecting a casing.
==Ammunition==
The M73 used 9.5x60mm ammunition, which was a full sized rifle round utilised in battle rifles and or general purpose machine guns. The round was larger than most cartridges used in infantry rifles; this meant that the M73 suffered increased recoil than lighter machine guns such as the M62 light machine gun. However, this increased mass gave it increased kinetic energy, allowing it to impart significantly more damage to the target than the 7.62x51mm NATO round, as well as having increased range and muzzle velocity. These characteristics made it suitable for use in the M73 machine gun.
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. 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 M73'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 M73 to deal noticeably increased damage to the target in terms of kinetic effects; this also markedly improved its armour and shield penetration characteristics. The 9.5mm 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 M73's internal mechanisms, 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.
The 9.5mm bullet was actually embedded inside the solid propellant 'block'; it, 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 9.5x60mm ammunition available to the UNSC, each differing in usage and effects. This varied from the simple and cheap to the complex and expensive to manufacture. The most commonly issued and utilised round was the M636 S/AP-HE, or Shield/Armour Piercing-High Explosive, which was a balance of potency and cost. Specialised ammunition, such as the M635 SAP-HEI, was generally only issued to high priority units and special forces, while forms such as the M603 Jacketed Hollow Point specialised to a degree where they were limited in their effectiveness, and as a result were not commonly issued.
*: The former standard round issued with 9.5mm calibre rifles, the M634 HP-SAP (or High-Powered Semi-Armour-Piercing) was largely superseded by more advanced rounds. The round featured a lightweight ballistic cap, which deformed on impact with the target and offered superior aerodynamic properties to the projectile in flight. The bullet's exterior was composed of lead, which fragmented and expanded outwards upon impact with a target. The internal penetrator was comprised of tungsten carbide alloy; between this and the ballistic cap was a small space or 'hollow', into which the penetrator would force itself upon impact with the target (this also expanded and fragmented the bullet's lead jacket, causing expansive and grievous wounds). Later iterations of the round also featured a polymer which coated the bullet, acquiring an electric charge in flight and assisting in shield depletion.
*M635 Semi-Armour Piercing-High Explosive Incendiary: The M635 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. However, the round's complexity in design made it expensive and its use was usually reserved for special forces and select few line units.
*M636 Shield/Armour Piercing-High Explosive: The M636 Shield/Armour Piercing-High Explosive, also known as S/AP-HE, was the most common and effective type of ammunition employed in the 9.5x40mm 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 milisecond 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.
*M637 Jacketed Hollow Point: The M637 Jacketed Hollow Point (JHP) 
*M638 Tracer: The M638 Tracer
*M639 Armour Piercing High Explosive: The M639 Armour Piercing High Explosive round 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.
*M640 High Explosive Squash Head: The M640 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==
*M73A1- the M73A1 was the initial production model that saw limited distribution. From 2543 all produced M73s were of the A2 variant, with existing A1s being upgraded to match.
*M73A2- The M73A2 addressed many features lacking on the original M73. The upper receiver saw the addition of a fixed rail that could mount a range of compatible optics, and doubled as a carrying handle. The A2 also received a removable rail interface system on the forward handguard, with three rails allowing attachments to be fitted to either sides or the bottom of the machine gun. These could be removed and replaced with the original A1 handguard for more comfortable holding, as these rails were often left unused. The underside of the weapon was furnished with a rearwards-folding extendable bipod, allowing increase stability when firing. This bipod could be replaced with a tripod for further stability. Finally, the M73A2 saw its firing mechanism strengthened compared to the A1 variant, which improved reliability in fully automatic mode, as well as a heavier barrel which reduced the effects of overheating.
*M73A2S- The M73A2 'S' model was a shortened version of the standard M73A2, modified to reduce size and weight. The A2S had its outer barrel length reduced considerably as well as a compact adjustable stock, making it more suitable for urban and jungle environments and orbital assaults performed by ODST units.
*M73A2A- The M73A2A was based on the standard A2 model M73, adapted for operation by personnel from aircraft. The A2A was equipped with a spade grip instead of the full stock to make airborne firing easier, while lacked the standard flash suppressor and integral bipod. The underside of the weapon was also modified so that it could be fitted to aircraft weapons mounts. A modification kit was issued alongside the weapon which included a full stock to replace the spade grip, a standard pistol grip and trigger assembly, an integral bipod and a flash suppressor, which meant the weapon could be quickly modified for use by downed aircrew.
*M73A2T- The M73A2T was designed for use as a mounted secondary weapon on armoured vehicles, such as main battle tanks. Unlike the A2A, it retained all the features of the standard model such as the pistol grip, full stock and bipod, as the increased likelihood of use by dismounted vehicle crew was recognised. Instead, the A2T was modified so that it could be fired either manually or electronically from inside the vehicle, via an Overhead Weapons System. The A2T was fitted to a quick detach mount meaning it could be quickly removed by the crew, should they be forced to bail out and take the weapon with them. This system allowed for a dismounting crew to retain some level of defence as well as providing close in support against enemy personnel while mounted. The A2T, like the A2A, was commonly fed by a much longer belt held in a larger box than the infantry version. Also like the A2A, the underside of the weapon was modified so that it could be fitted to weapons mounts.
==Gallery==
Image:M73 GPMG.png|An early-production M73 A1 model, which lacked rails and an integral bipod.
Image:M73A2_machine_gun.png|An M73A2 machine gun with forward rail interface system replaced with the A1-style handguard.
Image:M73A2_machine_gun_mod1.png|An M73A2 with bipod folded down and extended, and an ACG telescopic sight.