M990 electrothermal-chemical smoothbore cannon
From Halopedia, the Halo wiki
Unlike the M512 cannon mounted on the M808, or even the M310 cannons mounted on the M850 Grizzly, the M990 fires a much larger 150mm shell, and is capable of firing guided munitions and projected future advances in energetic propellants. ONI-operated Scorpions use an experimental shell, and Hannibal Weapon Systems has produced a prototype that uses a directed-energy weapon mounted within the barrel.
The M990 is extremely effective at penetrating heavy armor, including Type-58 Wraith tanks, and even Forerunner weapons such as the Z-510 focus turret. It is less effective against nimble aerial targets, such as the Type-54 Banshee or the Z-1800 Phaeton, but is still easily capable of penetrating their armor.
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