UNSC Heavy Weaponry

The United Nations Space Command utilised an array of heavy weapons for its vehicles and starships, ranging from conventional cannon and missile launchers to railguns, Magnetic Accelerator Cannons and nuclear missiles. Much of the UNSC's armament underwent extensive changes between the end of the Human-Covenant War and the turn of the 27th century, as , and reverse-engineered Forerunner technical assistance resulted in dramatic changes in the UNSC's own technological prowess.
==Cannons==
===M3706 20mm autocannon===
The M3706 autocannon was a belt-fed, six-barrelled, 20 millimetre autocannon that filled a wide range of roles and mission profiles. In service since 2494 with continual upgrades, the M3706 was could be mounted across a wide variety of platforms including the F/A-352B Longsword strike starfighter, the M901 Marksman air defence vehicle, Mantlet Air Defence System, and UNSC Navy warships. In the latter usage, the M3706 operated as the last line of defence in an air defence system, providing close-in protection against enemy aircraft and guided weapons. In Navy usage it was retired relatively early on during the Human-Covenant War in favour of the 4.1 inch Mark 22 naval gun, on account of the former's lack of effectiveness against Covenant starfighters and the absence of a missile threat from Covenant weapons. The M3706 also saw use as part of the Mantlet Air Defence System, a fully autonomous, self-contained unit which comprised the autocannon, magazine, RADAR system and processor. The Mantlet was capable of anti-vehicle and anti-personnel fire, but was primarily tasked with air defence against fast moving targets, and could be networked for greater situational awareness. 
The M3706 autocannon possessed six barrels, which rotated through external power and distributed the thermal energy created through firing equally, allowing for a rate of fire of 6,500 rounds per minute. The autocannon was chambered for the 20x128mm cartridge.
===M3911 30mm autocannon===
===1.9 inch mark VI naval gun===
The 1.9 inch mark VI naval gun in UNSC Navy service, was a fifty millimetre autocannon in service with the UNSC Navy. Mounted in fast-traversing, high-elevation quad turrets, the Mark XII acted as a close-in weapon system to defend vessels against against starfighters, missiles and other ordnance at short range, destroying them with accuracy and high volume of fire. Although moderately effective at destroying enemy single ships, the turrets lacked the firepower to deal damage to Covenant warships. The Mark XII augmented the M3706 in the close-in weapon system role, capable of a lower rate of fire but with significantly more damaging terminal effects. It was replaced in Navy service from 2651 onwards by the M502 railgun.
===4.1 inch Mark XXII naval gun===
The 4.1 inch (105mm) Mark 22 naval gun was a starship gun system equipping all UNSC vessels from the Mako-class corvette to the Trafalgar-class supercarrier. The 4.1 inch calibre had been in service since the UNSC's very beginning, although was by the 2500s relegated to secondary status as MACs and missiles advanced in the previous century. The Mark 22 was used by the UNSC Navy as a general purpose light armament, filling several roles. Mounted in pairs in armoured, fast tracking dual turrets, the Mark 22 was used primarily for point defence against starfighters and incoming ordnance, their relatively large calibre capable of overwhelming most Covenant single ships, though they also saw use in combat against light warships and ground targets. The Mark 22 in this way acted as a multirole light armament which could effectively be turned on any target, providing each vessel an increased degree of flexibility in combat. The gun could function in both offensive and defensive roles, directly assaulting targets or laying down supporting fire to assist allied vessels, or projecting a defensive 'screen' of anti aircraft fire using airburst munitions or other exotic types. The Mark 22 was gradually phased out from 2562 onwards by the 6.1 inch Mark 12 naval gun system, which utilised the same gunhouse and mount.
===M1105 120mm cannon===
==Railguns==
====M477 15mm Railgun====
====M481 20mm Railgun====
====M490 30mm Railgun====
The M490 railgun was a 30 millimetre automatic weapon used by a range of UNSC vehicles including armoured fighting vehicles, and fighter craft such as the F-371 Halberd. When used by fighters, it functioned as a general purpose light armament which could engage a range of target types effectively. These medium calibre railguns had a rate of fire of seven thousand rounds per minute and accelerated solid projectiles to fifteen thousand metres per second (15km/s), and were unsurprisingly effective against shielding and armour. Unlike many UNSC spacecraft, the weapons did not possess dual feeding, as a result of the complicated power and ammunition requirements, meaning the guns could not be fired from a selection of two ammunition types. However, each cannon was commonly loaded with different munitions types to imitate this effect; in addition, the weapon's firepower rendered the need to select a specific ammunition type largely obsolete.
====M502 50mm Railgun====
The M502 was a fifty millimetre railgun in service with the UNSC Navy, used as the last line of defence for warships and installations against enemy ordnance and starfighters. The M502 was developed as the last line of automated weapons defence against anti-ship missiles, including hypersonic and slipspace-capable models, and enemy single ships, including interceptors, bombers and boarding craft. Functioning as the final layer of a vessel's air defence systems, the M502 was a close-in weapon system responsible for tracking, engaging and destroying extremely fast-moving enemy targets that pose a threat to the ship or station. The M502 was individually capable of a cyclic rate of fire of 5,000 rounds per minute, giving the quadruple mount turrets they were most commonly used in a combined rate of fire of twenty thousand rounds per minute. The M502 was capable of accelerating solid slugs to a muzzle velocity of almost twenty thousand metres per second (20km/s), with devastating terminal effects for targets such as starfighters and missiles. This muzzle velocity gave it an effective range of approximately two hundred kilometres, and a maximum effective range of eight hundred kilometres, at which it was capable of harassing fire. Control of a ship's network of M502s was accomplished by a shipboard AI, either 'smart' or 'dumb', under the command of weapons officers on the bridge. Individual M502 systems possessed their own fire control radar, allowing them to lock onto and track their own individual target, though this was at all times augmented by the ship's far more powerful central search and fire control radar systems, which possessed greater range and resistance to electronic countermeasures. Intelligent management of the total network of M502 railgun turrets available to a vessel could be prioritised by any shipboard AI or central targeting processor, allowing for the most efficient engagement of targets during an intensive attack.
====M511 90mm Railgun====
====M521 120mm Railgun====
==Rocket and Missile launchers==
===M691 rocket launcher===
The M691 rocket launcher was a hexagonal, 15-tube rocket launcher, used for the 90 millimetre ASR-62 Thor WAFAR (wrap-around fin aerial rocket). The M691 was compatible with UNSC hardpoint systems, and was most commonly mounted to vehicles engaging in air-to-ground combat and close air support, such as the MV-14B Hornet, though could also be fitted to fighter craft, such as the F/A-352B Longsword strike fighter.
==Magnetic Accelerator Cannons==
===M68 Magnetic Accelerator Cannon===
===M72 Magnetic Accelerator Cannon===
===Mark III Light Magnetic Accelerator Cannon===
The Mark III Light Magnetic Accelerator Cannon, known officially as the Mark 3 MAC/L, was a heavy cannon used as the primary armament for frigate and destroyer classes such as the Remembrance- and Daring-class frigates and the Warrior- and Vanguard-class destroyers. The vessel replaced older, Human-Covenant War marks and was designed specifically for compatibility with the UNSC's new generation of high yield fusion reactors. Generally, Frigates carried one MAC while destroyers were equipped with two, a notable exception to this being the Vanguard-class light escort destroyer. As the primary armament for small warships, this relatively small cannon ran between half and two thirds of ship's overall length. The Mark III had a calibre of 5.5 metres (5500mm) and accelerated a 600-ton solid metal projectile to speeds exceeding half the speed of light (149,896,229 metres per second), giving it superb accuracy and terminal ballistic characteristics over extreme ranges. This was significantly faster than war-era MACs and was achieved through the use of reactors that were hundreds of times more powerful, as well as a larger number of capacitors and power recyclers which captured the residual energy from the capacitors after discharge. The impact energy alone from the slug travelling at such a velocity equated to 1.61 teratons of TNT, or 6,740,663.84 petajoules of released energy; although this would vary slightly depending on the type of slug. 
This energy was sufficient to completely destroy most Covenant vessels of cruiser tonnage and below with just two hits; the first depleting shielding and the second hitting and disintegrating the ship itself. In microgravity environments the slug maintained close to its muzzle velocity for as long as it travelled uninterfered with by the force of gravity; this, coupled with its high initial velocity, meant that the MAC could strike at targets quickly, accurately and over long ranges. As a result of the new, Covenant technology-derived reactors used by the UNSC, the MAC could fire far more shots on a single charge and then charge up again in a fraction of the time previously possible. This dramatic increase in rate of fire, coupled with the weapon's increased firepower, made even the smallest UNSC vessels a serious threat to Covenant ships of similar and higher tonnage.
Several types of slug were available to the Mark 3 MAC, providing a range of function and effects. The slugs' official designations include the 'Kinetic Penetrator' prefix to denote their use in MAC systems, distinguishing them from small arms ammunition.
*Kinetic Penetrator, Tungsten: This slug was a simple, single-compound slug composed of superhard Tungsten Carbide. This material made its effects simple in that they were limited to penetrative power, although in this area the slug was exceptionally capable. The KPT slug was often used to supplement the more capable but more expensive KCP slug.
*Kinetic Penetrator, Ceramic: The most commonly used slug, in addition to KPT, was a Kinetic Composite Penetrator slug, or KCP. The KCP slug was formed of a ceramic and CVT/Titanium alloy, metal/ceramic metal matrix composite, in alternating, concentric layers. This had superior penetrating properties than either ceramic or metal alone, and was comparable in density to titanium yet retaining a hardness exceeding that of tungsten. It was commonly used in conjunction with the cheaper KPT slug.
*Kinetic Penetrator, Reactive: The KPR slug was formed of depleted Uranium, or DU, which had penetrating qualities close to those of the KPC slug, yet was incendiary. This meant that the slug ignited on contact with the target and entered it with significantly increased secondary effects.
*Kinetic Penetrator, Fragmentation: The KPT slug was composed of a 'TC3' alloy, made up of tungsten, cerium and copper carbide, which had a similar density to DU, without radiological effects, though retaining an incendiary function. The slug would ignite on contact with the target and carry into it incendary material, which increased secondary effects considerably, making it more useful for larger targets that could withstand multiple strikes. The slug was constructed to decelerate considerably and fragment after impacting the target, spreading large fragments of incendiary metal scything through the ship's interior.
*Kinetic Penetrator, Multiple: The KPM slug was in actual fact a 'bundle' of three smaller slugs fired at the same time. These slugs then travelled to the target parallel to each other but with space between them, creating three smaller impacts rather than one larger one, and focusing kinetic damage in the area between them, causing much larger overpressure damage that would travel completely through the target, even if the slugs did not. This made the KPM slug more useful for smaller and weaker targets.
===Mark VII Light Magnetic Accelerator Cannon===
The Mark VII Light Magnetic Accelerator Cannon, known officially as the Mark 7 MAC/L, was a heavy Gauss cannon that was primarily used as the primary armament for light frigates and corvettes during the latter stages of the Human-Covenant war. The weapon system was phased out usage following the conclusion of the Human-Covenant War, with most warships having the weapons system replaced with the larger Mark 3 MAC/L, with the notable exception of the Drake-class corvettes, which was unable to accommodate the larger weapon system, and instead had installed systems extensively modernized, and additionally modified with better capacitors and coils. 
Designed midway through the Human-Covenant War, the Mark 7 MAC/L had a caliber of 3.18 metres (3180mm), and was designed to fire a 300-ton tungsten round at speeds of 0.03c - or approximately 3% of the speed of light. With overhauls to the Drake-class and overhauls to the reactor, the speed of the projectile were increased to 0.37 c, which equated to 37% of light speed. This exponentially enhanced the damage inflicted by projectiles on targets.
As the Mark 7 MAC/L was largely phased out of service and used by only one active class of warship, the only remaining stocks of 3180mm slug that remained was the Mark IV Kinetic Projectile, fragmentation. It was comprised of a small, dense depleted uranium kinetic energy penetrator that was designed to punch through the entirety of a target to inflict catastrophic damage. Shrouded this DU "bullet" was a magnetically-conductive layer of ferric-tungsten that allowed the projectile to be fired from M2 MAC/L cannon, and was designed to immediately begin to fragment into millions of shrapnel shards upon impact in order to perforate the maximum number of decks, crew and subsystems.
===Mark XII naval gun===
The Mark XII naval gun (officially called the 6.1 inch Mark 12 naval gun system) was a UNSC naval gun system designed for general purpose fire support duties. The Mark XII was mounted in dual turrets and fitted to all UNSC vessels of frigate size and above (with the addition of the Bow-class corvette), replacing the in the general purpose fire support role. The Mark XII was a versatile weapon providing a multi-role fire ability, capable of fire support, anti-surface, anti-ship and anti-air fire missions, and functioned by magnetically accelerating a 155 millimetre ferrous slug to half the speed of light.
When used in an anti-air role, the weapon's high accuracy and terminal effect per round compensated for its low rate of fire compared to traditional close-in weapon systems. This enabled it to engage single ships efficiently while also maintaining a number of additional taking on stronger targets and a in wider range of roles. In an anti-ship role, the naval gun was more powerful than conventional antiship missiles, as well as faster, giving it a strong light anti-ship role. The gun could be directed to fire at vulnerable external targets such as shield generators and weapons, overwhelming shields with powerful localised fire and disabling enemy systems before the main shields were disabled. Dependent on range, the Mark XII was also capable of directing fire through openings in enemy shields caused by active enemy weapons, in this way destroying point defence lasers, plasma torpedo launchers and large energy projectors. The Mark XII could also function in an anti-surface role, engaging ground targets using targeting information provided by allied ground units. The gun systems' unprecedented accuracy and firepower was achieved at great cost effectiveness, whereby a single slug would take the place of a multi-million credit missile.
The naval gun acted as a multirole light armament which could effectively be turned on any target, providing each vessel an increased degree of flexibility in combat. The gun could function in both offensive and defensive roles, sometimes simultaneously, directly assaulting targets or laying down supporting fire to assist allied vessels, or projecting a defensive 'screen' of anti-aircraft fire using advanced airburst munitions or other exotic types.
Target selection and tracking was handled by the shipboard AI, either directly or by the automated fire control system under the AI's supervision. The Mark XII's lightweight, armoured turrets had a total elevation of 165 degrees, and could traverse fully in under six seconds. Including the gunhouse, magazine, fire control room and power room, the weapon system full system occupied five decks. The general design for the Mark XII was based on the , though thoroughly updated and modernised for a magnetic system. The biggest visual change was the redesigned gunhouse, intended to reduce radar cross-section. The UNSC's vastly improved reactors, engineered with Sangheili and and Huragok assistance, meant that the Mark XII could achieve a cyclic rate of fire of 50 slugs a minute. A range of ammunition types were available in the 6.1 inch calibre including Kinetic Penetrator Tungsten, Ceramic, Reactive, Fragmentation and Multiple slugs.
===Mark XIII naval gun===
The Mark XIII naval gun, officially called the 12 inch Mark 13 naval gun, was a heavy naval gun designed to supplement the lighter Mark 12, and provide heavy fire support function to ships of cruiser tonnage and above. The Mark 13 naval gun accelerated a 20-ton projectile to speeds exceeded half the speed of light, resulting in terminal kinetic energy of nearly 265,000 Petajoules, and was capable of fire-support, anti-surface, anti-ship and limited anti-air fire missions. A range of ammunition types were available in the 12 inch/305mm calibre including Kinetic Penetrator Tungsten, Ceramic, Reactive, Fragmentation and Multiple slugs.
===Mark XIV Medium Magnetic Accelerator Cannon===
The Mark XIV Medium Magnetic Accelerator Cannon, also referred to as the M-MAC, was a larger gun for use on ships of cruiser tonnage and above.
===Mark VI Heavy Magnetic Accelerator Cannon===
supercarriers and ODPs
==LASERs==