The chaingun in Doom 3 is a good mid range weapon with a high rate of fire. It is considerably more powerful than its classic Doom counterpart. While the old chaingun was at best, a rapid fire weapon that was clearly inferior to the plasma rifle, Doom 3's chaingun is more on par with the plasma gun.
Very powerful; good belt size; medium accuracy with spread; heavy recoil; slight wind-up time. This should be used for Cacodemons, imps, (pinky) demons, Mancubi, and Hell Knights. Ammo counter is located on the weapon.
The chaingun is the gun-of-all-seasons, to say the very least. High accuracy and rate of fire make it a good weapon to use in anywhere from close to far range situations against almost any low to mid-range foe. Even a Hell Knight can be taken down with this supreme piece of military technology.
Albeit it has stupendous firepower at any length, both short and long range use is discouraged. Short range attacks mean possibly being taken down by a melee attacker (such as a tentacle Commando zombie) in a situation where you want to lay down a lot of pain in a split second (say, with a shotgun).
Despite its incredible accuracy, long range combat is also discouraged because the chaingun has no splash effect damage, meaning that unless the target stands perfectly still or you just fire blindly in the targets movement range (which means wasting ammo), you are not going to do much damage.
The best situation is a relatively slow monster that affords enough time to be taken down and requires a steady stream of fire to be taken down. Pinkys and Cacodemons come to mind, as does Vagary. Also, a group of Trites or Lost Souls can easily be taken down with some blind firing.
When using the chaingun, take into consideration its two flaws. It does have a warm up period, albeit short, that can allow a quick opponent a chance to throw in a melee attack and soften you up for the kill. Secondly, only 400 rounds can be carried at any time for the chaingun, and its high rate of fire can mean running out of ammo very quickly.
While the pistol, machine gun, and chaingun all technically fire bullets, they do not share the same ammo pool as in other games.
The chaingun first appears in the Martian Buddy storage locker in Administration. It can also be found - along with several ammo belts - in the really dark areas of Alpha Labs Sector 2 through which Edwards, the lantern man, will guide the player.