The BFG 9000
It's known by many names...
Bio Force Gun. Big Fragging Gun. Big Freakin' Gun. Big Fvcking Gun.
But it's pretty effective against the denziens of Hell, as our beloved Space Marine (aka Doomguy) finds out. One shot from this bastard and you'll turn many a demon into red paste.
Respect the awesomeness and badass-ness that is the BFG 9000.
The BFG9000 appears as a large, solid metal gun which fires large balls of green plasma. For general purposes it can be considered the most powerful weapon in the game; it is capable of destroying nearly any player or monster with a single shot.
The abbreviation BFG stands for big f-cking gun, as explained in section 14 of the Doom Bible. Other expansions of the name were attributed to it before that document was made public, notably "big fragging gun". Characters in the Doom novels refer to the BFG as a "big freaking gun". In the Doom movie, BFG officially stands for "Bio Force Gun", although Sarge calls it a "big f-cking gun".
The BFG first appears in a secret area of E3M3: Pandemonium, and again in secret areas of three of the next four levels. It is eventually found in a non-secret area in the expanded version, in E4M5: They Will Repent. When picked up, the BFG contains 40 energy cell units (80 on the "I'm too young to die" and "Nightmare!" skill levels). It expends 40 energy cell units per shot.
When the trigger is pulled, there is a pause of 30 tics (about 0.857 seconds) before a green and white plasma ball is ejected. If the large plasma ball hits a solid object, it explodes and causes 100-800 hit points of damage to the target, in round multiples of 100.
After a further pause of 16 tics (about 0.457 seconds), additional damage is calculated: 40 invisible tracer rays are emitted by the player in a cone-shaped area (about 45° half-angle) in the direction the plasma ball was fired. (If the player has turned around, the direction of the rays does not change — they are still traced in the direction of firing of the original plasma ball. If he has moved to another location, their origin moves along with him.) Each ray causes 49-87 points of damage if it hits a solid object within 1024 map units. Even cyberdemons and spiderdemons, which are immune to blast damage, are affected by these rays.
Therefore, the minimal damage of the weapon is 49 points (if an object is hit by one ray and not the plasma ball) and, hypothetically, the maximal damage is 800 + (40 × 87) = 4280 points (if the plasma ball hits an object for full damage and all 40 tracers also hit the object for full damage). However, even should all 40 rays and the energy ball hit a single target, that much damage can still never actually be inflicted due to the periodicity of the simplistic pseudorandom number generator used by the Doom engine.
Contrary to section 3H of the BFG FAQ, the tracer code does not include horizontal auto-aiming (although, like any bullet attack, each tracer can auto-aim vertically).