Heavy plasma cannon
From Halopedia, the Halo wiki
Heavy plasma cannon is a general term referring to multiple types of Covenant plasma weapon. Essentially a more powerful version of the medium plasma cannons mounted on T32 RAVs, the heavy plasma cannon fires bolts of superheated plasma and can be found mounted on many Covenant vehicles, including T-25 Spirits, T-44 Phantoms, T-52 Phantoms, T-27 Banshees, T-31 Seraphs and Type-47B Scarabs.
The heavy plasma cannon is not a single type of weapon; instead, there are multiple weapons with different properties that fall under the category.
In Halo: Combat Evolved, the gun is outfitted on Spirit dropships and is very similar in shape and in the ammunition it fires to the Shade turret. The cannon has the shape of a ball with three prongs attached that fire a burst of needle-like purple plasma bolts. The "ball" seems to levitate below a fixed casing structure that has two wings on the side. The gun also has limited mobility, and cannot fire directly below or above it and is also limited in its sides. The gun can, however, fire through the body of the Spirit when an enemy is on top of it, and is likely just a glitch.
In Halo 2, the cannon changes its appearance and is now outfitted on Phantom dropships. The gun seems more mobile and has multiple "joints". The ammunition it fires has also changed. It now fires big red bolts of plasma at a slower rate, but these do more damage.
In Halo 3, the design of the Phantom-mounted cannon is mostly the same, but the bolts it fires are slightly different, being less powerful. The cannon also seems to fire faster than in Halo 2. A larger variant appears on the Scarab, serving as an anti-aircraft weapon.
In Halo: Reach, a type of heavy plasma cannon is again outfitted on both Spirit and Phantom dropships. A different version is mounted on the space version of the Banshee, the Type-27 Exoatmospheric Multi-role Fighter and on the Seraph fighters.
The version mounted on the Phantom is very similar to the one in Halo 2 and Halo 3, and the one on the Spirit closely resembles the one from Halo: Combat Evolved, the only difference being that the cannon sits higher in the casing. The ammunition that both these versions fire has changed; the bolts now fly in arcing trajectories and are similar to the bolts fired from the Concussion Rifle, being very powerful but also very slow.
The heavy plasma cannons mounted on both the Space Banshees and on the Seraph fighters greatly vary in shape (the ones mounted on the Space Banshees are very elongated, and the ones on the Seraphs are very small and build into the "wings"), but both fire the same type of ammunition, smaller sized blue plasma bolts, which are fired faster but do less damage than the normal "concussion" version of the heavy cannon mounted on the Phantoms and Spirits.
Also, the Phantom gunboat, a new space version of the Phantom introduced in Halo: Reach is equipped with two new space combat versions of the cannon, one chin mounted gun with three long prongs at the end, and multiple smaller cannons mounted on the sides. Even though the chin-mounted cannon and the side cannons look different, they fire the same type of big blue plasma bolts, similar to those from the normal "concussion" version.
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