The Chainsaw is an extremely powerful close-quarters combat weapon, based on the classic weapon. The chainsaws in the game were sent to the Mars Base by the Mixom Corporation by mistake (or as a "divine intervention"); the original order was for jackhammers. Humorously and ironically, in PDAs, a worker discusses how useless chainsaws are on Mars; this and the story behind their delivery are perhaps in answer to the inconsistency (from the original Doom) of having a chainsaw in a place where there are no trees.
Known commercially as Mixom's Beavertooth, it is a chainsaw powered by some kind of internal combustion engine (as seen by the smoke it generates when started). The player may aim it like most other weapons but it is still only effective in melee combat. The length of the Beavertooth chainsaw is 30" according to the boxes the saws came in.
The chainsaw does not appear in Doom 3: Resurrection of Evil or in Doom 3: The Lost Mission, the bonus campaign featured in the Doom 3: BFG Edition package. It seems to have been replaced in these campaigns by the Grabber. It is not available in PC deathmatch but in the Xbox version the chainsaw replaces the plasma gun.
Being a melee weapon, the Chainsaw does not require any ammunition to function, hence it is able to run indefinitely. However, it also takes a noticeable amount of time to rev up the chainsaw for usage, hence players may want to take this fact into consideration, particularly in close-quarters combat.
The chainsaw can be used, as a last ditch attempt, to survive when out of ammunition or as a means to dispatch zombies, due to the fact the weapon has effectively infinite ammunition like its classic counterpart. It is also particularly effective against newly-spawned monsters; like Imps (as they require a short amount of time to activate), Arch-Viles (since they are vulnerable when summoning monsters), and Wraiths whilst teleporting. The Cacodemons' habit of flying toward the player also makes them vulnerable to being gnawed. This weapon is also useful against riot-shield zombies, as the blade can cut through their bulletproof shields.
Utilizing hit and run tactics is also recommended as the chainsaw deals massive damage once it hits the target, and is guaranteed to slice enemies like a hot knife through butter. Multiple strafing is advised against enemies with long range attack capabilities.
However, any enemies beyond that should be approached with caution. Usage on powerful enemies (such as Mancubi or Hell Knights) is not recommended as they will counter-attack the player, potentially causing heavy damage. Ironically, this is also the case with Demons, who were singularly vulnerable to the chainsaw in the original Doom. In Doom 3, the demon can now "shake" the player, disturbing the weapon aim in the process. The chainsaw is also not ideal for agile-small posture-flying enemies like lost souls. If these conditions are unavoidable, it's wise to step back in line of the lost soul's attack pattern as it will go, without thought, directly straight into the player's already spinning blade to be cut up to ribbons. Overall, despite the new disadvantage with the Demons, the Chainsaw is a considerably more useful weapon in Doom 3 than in the classic games, where it was only really useful against Demons (and Spectres), and a decidedly inferior choice against all other monsters.
It should also be noted that the chainsaw's motor creates a large amount of noise while it is being wielded, which can prevent the player from hearing the noise cues generated by nearby enemies intended to warn the player of their presence. This can be circumvented by quick-switching to the chainsaw using the weapon's select key, rather than having it equipped while exploring.