The heavy weapon is much more powerful than your primary and special weapon, but you only get ammo for it when it spawns during a match, typically after 5 minutes of the match starting.
The Best Heavy Weapon PvP The Crucible Destiny
Machine Guns are without question the best heavy weapon in the Crucible in Destiny. They do a lot more damage than auto rifles and will let you easily win most engagements against enemies that don’t have heavy ammo. If you know an enemy has heavy ammo it is always better to run away than to try to fight them. The best machine guns are those with low recoil and high impact.
Rocket launchers are very situational. They are good when there is a crowd of enemies, but that rarely occurs. Even against two enemies it is often easier to kill them with a machine gun.