The Machine Gun, also known as the MG-88 Enforcer, is a weapon introduced in Doom 3. It is essentially an assault rifle, that has a large magazine size, high rate of fire, and great accuracy. It is useful against most demons up to the Cacodemon and especially useful against Lost Souls. Its default key is 4. The Machine Gun uses ammunition from Machine Gun magazines. It has a very high rate of fire, but less damage than the pistol. It should be used to mow down hordes of lesser demons or zombies. It can also be used for long-range work against enemies who are hiding behind cover. Take note that the ammo counter is on the weapon itself and not on the game HUD - similar to the chaingun, plasma gun and BFG 9000. In Doom 3: Resurrection of Evil, it is now the main starting weapon in Multiplayer while the pistol acts as the Sidearm.
It is first found in Mars City Underground after unlocking the second storage locker from T. Brooks' computer. If not, it can be found in Mars City off of one of the Z-Secs.
It is probably based on the FN P90 personal defense weapon, with UAC design. While ironsights are present on the MG-88, during close examination it only houses the rear sight notches up the front, with no front sight present on the gun, making them usless for anything other than close-quarters. There are also no optical sights, or electronic sights present on the MG-88; it instead houses an Uplink Module inside the ammo counter, to be used with the Mars Security and Marine helmets. Were it to be reproduced in real life, the weapon could not be accurately aimed at longer ranges, making it nothing other than a hip-fired, close quarters, somewhat accurate "spray and pray" weapon. It should be noted that firing without using sights in reality is regarded as useless and inadvisable unless at close quarters.
Machine guns are good in medium to short range situations against lightly armored foes. Zombies make the best targets, being slow and weak. Trites are also best picked off with the machine gun, as well as Cherubs and Lost Souls. The machine gun is most effective against Z-Secs (aside from those armed with Riot Shields) as it takes down the lightly armored foes quicker than the pistol due to its high rate of fire and is more accurate than the shotgun at medium-to-long range. In a tight spot, machine guns are good versus Maggots and Imps as well. On lower difficulties, a player with a good aim can easily deal with a Demon. The machinegun also has a unique buzzer alarm which warns players that their clip is running short, once the player has been reduced to 10 bullets.
The weapon's rate of fire has two drawbacks. The first is that rapid fire causes the weapon's recoil to disrupt the player's aim. The second is that the weapon burns through its magazine quickly. When using the machine gun, it's recommended that you be near cover in case you need to reload during the middle of a firefight. Generally, the weapon is an ideal firearm to have at the ready whenever the player enters a new area but does not know what is waiting there.
- A machine gun was originally going to be used in the original Doom game. First-person sprites for it exist in both the 0.4 and 0.5 alphas. Its 0.4 appearance appears to have been a modernised MP40 (an obvious callback to Wolfenstein) but its later 0.5 appearance is similar to some sort of 7.62mm Assault Rifle.
- In the 2016 Doom reboot, the equivalent firearm of Doom 3's Machine Gun is the Heavy Assault Rifle.