M990 electrothermal-chemical smoothbore cannon

The M990 electrothermal-chemical smoothbore cannon is a vehicle-mounted heavy weapon in the service the primary armament of the M820 Scorpion, introduced into widespread service in 2557.

Design details
Unlike the M512 cannon mounted on the M808, or even the M310 cannons mounted on the M850 Grizzly, the M990 is an electrothermal-chemical gun, and is considered the single-biggest improvement on the M820 platform over its M808 predecessors.

According with its new operating principle, the M990 breaks legacy munitions compatibility with its predecessor models and fires a much larger 150mm shell, and is capable of firing guided munitions and projected future advances in energetic propellants. Some of the projected propellant advances are already in service among the fleet of select Scorpion variants operated by the Office of Naval Intelligence's security forces, which use an experimental shell and the latest energetic propellants.

The XM820B4 Scorpion, an experimental model produced by Hannibal Weapon Systems, uses the M990 ETC gun's hull and structure as a base for the integration of a fast-discharge laser weapon system. Aesthetically, this weapon has few outward differences than the standard M990, though is functionally a directed energy weapon rather than a conventional tank gun.

The M990 is extremely effective at penetrating heavy armor, including tanks, and even Forerunner weapons such as the Z-510 focus turret. It is less effective against nimble aerial targets, such as the or the Phaeton, but is still easily capable of penetrating their armor.

List of appearances

 * Halo 5: Guardians