The Chainsaw is an extremely powerful melee weapon, based on its classic iteration. 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 Doom 1) of having a chainsaw in a place where there are no trees.
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.
After obtaining the Chainsaw, the player can switch between it and the Fists using the "1" key (default fists key).
Known commercially as Mixom's Beavertooth, the Chainsaw is powered by some kind of internal combustion engine (as seen by the smoke it generates). 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.
As its only usage is limited to melee, 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 mowed. 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 tear through enemies. 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. The chainsaw is also not ideal for the smaller "pest" 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 weapon.
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.