The Plasma Cannon is the 8th and 1st of the heavy weapon which comes with 15 ammo, 2 upgrades can be bought: movement speed and firing rate.
The Plasma Cannon delivers high damage, has a slow firing rate, and comes with 15 ammo.
This weapon should always be bought after acquiring the Rifle. This way it ensures an advantage over other players and being OP. Rate of Fire is the only upgrade that should be purchased to become overly OP.