Alright, here's the weapons I've got so far.
*Note: Most of them fire almost abnormally high-powered ammuntion. This is partly because the guns themselves are huge, and partly because of the advancements in armor.
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Model: 15mm Autocannon
Manufacturer: UNSC
Type: Autocannon
Size: Shoulder Fired
Magazine Size: 24
Damage Per Hit: High
Fire Mode: Automatic
Ammunition Type: 15mm High-Explosive Rounds (60 Cal.)
Operation: Gas-Operated
Rate of Fire: Medium (120 Rounds per Minute)
Accuracy: Medium
Range: Mid-Long
Description: The UNSC Autocannon is a weapon designed for anti-materiel purposes. The high-explosive rounds are effective at piercing armored targets, and have a devastating effect on infantry. The 15mm shells are lethal at close to mid range, but their relatively low velocity makes them easier to avoid at longer ranges.
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Model: HK-36 Combat Rifle
Manufacturer: UNSC
Type: Combat Rifle
Size: Shoulder Fired
Magazine Size: 30
Damage Per Hit: High
Fire Mode: Semi-Automatic
Ammunition Type: M225 Semi-Armor-Piercing High-Explosive
Operation: Gas-Operated
Accuracy: High
Range: Mid-Long
Description: The HK-36 has become standard issue among UNSC forces, phasing out the BR55HB SR in 2557. While it lacks the famed burst-fire ability of the Battle Rifle, its ability to deliver high-powered rounds with deadly precision has made it quite popular among UNSC forces. It also has limited armor-piercing capabilities, increasing its effectiveness against lightly armored vehicles. They are generally issued with telescopic scopes capable of 3.5x zoom.
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Model: Grenadier Pistol
Manufacturer: Jiralhanae
Type: Self-Propelled Grenade Launcher
Size: Handheld
Magazine Size: 6
Damage Per Hit: Medium
Fire Mode: Semi-Automatic
Ammunition Type: 20mm Self-Propelled Grenade
Accuracy: High
Range: Close-Medium
Description: The Jiralhanae Grenadier Pistol is one of the most unique handguns to-date. Rather than firing standard ballistic rounds or plasma-based projectiles, the Grenadier Pistol fires miniaturized self-propelled grenades. While they lack the punch of the standard Brute Shot, their compact design makes them much more practical for general use. They demonstrate extreme effectiveness against infantry.
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Model: MA5X Assault Carbine ("Shredder")
Manufacturer: UNSC
Type: Carbine
Size: Shoulder Fired
Magazine Size: 75
Damage Per Hit: Low
Fire Mode: Automatic
Ammunition Type: 7.62x51mm NATO
Operation: Gas-Operated
Rate of Fire: Very High
Accuracy: Low
Range: 50 meters (155 ft)
Description: The Assault Carbine is the newest addition to the MA5 series, designed with Urban Combat groups and vehicle crews in mind, as the compact, lightweight design increases mobility in tight quarters. While the individual rounds are somewhat weak, its incredible rate of fire causes Marines to jokingly refer to it as "The Shredder". The Ma5X is fitted with a tactical rail for mounting the weapon's holographic sights, as well as numerous other attachments.