DOOM II: Hell on Earth

DOOM II: Hell on Earth

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The BFG 9000: How it Works
By Melon
The BFG is Doom's most famous weapon, but do you understand how it works? If you thought it was similar to a super-powered rocket launcher, then you're wrong, and this guide is for you. This guide will teach you what's going on behind the scenes and how to maximise the damage you deal with each shot, allowing you to kill a Cyberdemon in only two shots!
The way the BFG deals damage doesn't match up with the way you'd assume based on how it's presented. Most players assume that it's a super powerful rocket launcher, dealing lots of splash damage in a circle around where the big green projectile detonates, but the actual way it works is completely different. This guide will teach you all about how it works, and how to exploit these mechanics to the max.

Note: Specific numbers used in this guide come from the Doom wiki[].

The damage caused by the BFG occurs in two different ways: the giant projectile, and "splash" damage that occurs once the projectile detonates (this will be referred to as aftershock damage for the rest of this guide). We will deal with the specifics of these two damage sources separately.
The Projectile
Roughly one second after firing the BFG, a large green ball of plasma will be fired from it. This projectile travels fairly slowly and detonates against the first thing it hits. Despite the large animation that plays when it hits something, this plasma ball can only ever deal damage to a single enemy, the enemy the projectile hits. The actual damage dealt is a multiple of 100 randomly chosen between 100 and 800 inclusive (so 100 damage, 200 damage, etc.).

For reference, here is a list of all of the hit points monsters have, ordered from lowest to highest:
  • Zombieman (pistol zombie) - 20
  • Sergeant (shotgun zombie) - 30
  • Wolfenstein SS (Nazi in secret levels) - 50
  • Imp - 60
  • Chaingunner - 70
  • Lost Soul (flying skulls) - 100
  • Demon / Spectre (the pink melee monsters and their partially invisible version) - 150
  • Revenant (skeletons with homing missiles) - 300
  • Cacodemon (red flying balls) - 400
  • Pain Elemental (flying brown balls that shoot Lost Souls) - 400
  • Hell Knight (grey, throw green fireballs) - 500
  • Arachnatron (spiders that sahoot plasma - 500
  • Mancubus (fat, fire two fireballs at once) - 700
  • Arch-vile (yellow, throw you into the air with fire, ressurect monsters) - 700
  • Baron of Hell (red, throw green fireballs) - 1000
  • Spider Mastermind (giant spider with chaingun) - 3000
  • Cyberdemon (Shoots rockets) - 4000
As you can see, while the BFG projectile by itself has a chance to kill most enemies in one hit, it also has a chance to do very little damage that all but the weakest of enemies can survive. In fact, a direct hit from a rocket deals 148-288 damage to most enemies (the Spider Mastermind and Cyberdemon are immune to splash damage from rockets), so sometimes the BFG's plasma ball will deal less damage than a rocket!

Simply put, the actual projectile is not very dangerous and makes up only a small portion of the BFG's damage output, so it's not a big deal if your shot misses. What really counts is the aftershock damage, which is also the most complicated part.
The Aftershock
The most dangerous and least intuitive part of the BFG is the aftershock damage. This damage occurs almost half a second after the projectile hits something (if the projectile never hits anything because it flies away into the sky box, then this damage will never occur).

The actual aftershock damage is emitted from where the player is standing in a cone shape. The direction this cone is emitted is in the direction the projectile was fired at. This is important. Note that this is not the direction the player is looking in. It is also not aimed towards where the projectile detonated. If you, for example, fired a BFG ball towards the South according to the automap, then the cone of aftershock damage will fire to the South from the player's current position. If this is tricky to understand, then the following image may make it easier:

This means that the aftershock damage may not actually happen anywhere near where the BFG ball exploded, particularly if you fired it at a target that's far away. If a player assumes the BFG acts like a rocket, you might see them fire it at a distant group of enemies and then run away and hide. What usually happens in this case is that the aftershock damage doesn't hit anything and the shot only dealt the small amount of damage that comes from the projectile to a single enemy. This is exactly the wrong way to go about maximising use of the BFG. In practice, the BFG acts more like a super-powered shotgun.

So how much aftershock damage is dealt? The cone of damage is made up of 40 "hitscan" shots, each dealing between 49-87 points of damage. The cone is roughly 45 degrees wide. There is also a range limit to these shots but it's pretty large (1024 map units, for reference 32 map units is the size of the player's hitbox and the size of a typical teleporter platform). The theroetical maximum damage caused by the aftershock damage is therefore 3480, much much higher than the projectile. In practice, the total of the projectile and aftershock damage will deal a bit less than 3000 points of damage to a nearby and tightly clustered group of enemies.
Maximising the Damage of Each Shot
If we want to make the most out of our 40 cells used from each shot, we should aim for making the aftershock damage hit as many targets as possible. Because this damage is emitted in a cone shape, if an enemy is standing at point-blank range, it can potentially be hit by every single one of the 40 hitscan shots. While all of these shots are resolved in the same game tic, they are applied one after the other, so if one of them kills an enemy, the remaining shots will travel through the enemy's dead body and potentially hit more targets. This means that no shots are wasted by being absorbed by dead enemies.

When against a group of enemies, it might be wiser to stand back so you don't get slaughtered by the enemies and have the damage spread out amongst the group, but against single targets you should dive right in and attempt to get every aftershock shot hit the same target, potentially one-shotting the enemy. This tactic is guaranteed to one-shot every enemy in the game except for the Cyberdemon and Spider Mastermind.

Despite the theory showing that you can deal over 4000 points of damage this way, there are two things that make this maximum unreachable in practice, one is the limits of Doom's random number generator, the second is a bug called the BLOCKMAP bug that means that under certain conditions hitscan shots that hit a target don't actually register. The huge size of the Spider Mastermind makes this latter bug very likely to occur, meaning that one-shotting it is possible but extremely rare, you should always assume it's not going to deal enough damage in a single shot. In some source-ports such a ZDoom, this bug is fixed and so it's more common, but still requires luck. The limits of the random number generator makes one-shotting a full-health Cyberdemon impossible.

Because of the delay present in charging up the plasma projectile, having it hit the target, and another small delay until the aftershock damage occurs, you usually want to fire the BFG fairly early when moving towards the enemy, ideally the aftershock damage will trigger as soon as you reach point-blank range with the enemy. If you fire it too early, some aftershock shots might miss, and if you fire it too late, you run the risk of standing really close to the enemy for long enough for them to hurt you. There's no substitute for practice.
Killing a Cyberdemon in Two Shots
As the previous section shows, in order to deal enough damage to kill a Cyberdemon in two shots you're going to need to hit him with the BFG twice at point-blank range. Given the Cyberdemon fires rockets that will one-shot you under most circumstances, surely this is suicide, right?

Well no. It's actually far easier than you might think. Obviously it will take practice but there's a trick to it. When the Cyberdemon fires his rockets he always does so in salvos of three at a time. These are always fired one after the other with a very particular timing. With enough practice, you will be able to perfectly predict these and dodge them effortlessly every time. It should go without saying that having "always run" turned on really really helps (this is an option in pretty much every source port, usually toggled on using CAPS LOCK).

The trick is to stand fairly close (but not too close) and change directions between each rocket he fires. You want to fire your BFG when he fires his second rocket, and move in for the kill while you're dodging the third rocket. You want to deal the damage as soon as possible after the third shot so you can get in and out before the Cyberdemon has a chance to start another rocket savlo. Changing directions between each rocket makes it easier to stay pointing in the direction of the Cyberdemon with as little mouse movement as possible, making you more likely to have the cone fire at the correct angle as opposed to constant circle strafing. This is also the best way if you play only with the keyboard, as you don't need to turn at all, only strafe.

An easy way to practice is to load up E2M8 in the original Doom or MAP32 in Doom2. You can do this by starting a new game and typing 'idclev28' (Doom) or 'idclev32' (Doom2). Then type 'idfa' and you'll be given all weapons with full ammo.These levels both have nice big arenas with a cyberdemon in it. If you play on the easiest difficulty setting "I'm to Young to Die" then these levels will have no other enemies in them and you'll take half damage so if you get hit by a rocket you won't die straight away. Note that this will take a fair amount of practice. Not hours, but still a fair amount until you've got it down. Try staying around the Cyberdemon at various distances and practice just dodging rockets, changing direction between each rocket. Once you've got the dodging down the rest comes easily.

IMPORTANT NOTE: In the vast, vast majority of Doom levels you will play, two-shotting a Cyberdemon is a completely unnecessary risk. It is way way safer to not move all the way in to point-blank range and instead keep a little bit of distance. This will usually result in a 3-shot kill which is still very fast and ammo efficient, and improves survivability massively.

If a Cyberdemon ever turns against another monster then this is the perfect time to get a shot in while his back is turned and is a much safer way to deal lots of damage. Also if you are playing with fast monsters on (the -fast launch parameter or Nightmare! mode) then trying to two-shot a Cyberdemon is suicide and you're going to die. Don't even try it.
Plasma Gun vs BFG
When is it worth using the BFG? While the BFG can cause a considerable amount of damage it takes 40 cells to fire it. It should be obvious that with a big and nearby group of enemies where you are likely to have all of the aftershock shots hit a target, the BFG gives a considerably higher damage output for 40 cells, but what about individual targets?

The Plasma Gun will, on average, deal about 800 points of damage with 40 cells, so against most single targets, the Plasma Gun is more efficient. If you can get close, then Barons of Hell and Spider Masterminds are great targets, as are Cyberdemons (but they might not be worth the risk). However a key difference between the BFG and Plasma Gun is that the BFG deals most of its damage in one big burst, while the Plasma Gun requires that you stand out in the open for an extended period of time. Against Arch-Viles, you only have a short window of time to attack before you need to run for cover, so they are also perfect targets for a BFG blast, especially given that they have no melee attack and so are safe to stand next to.

The burst damage is also nice for those maps where standing out in the open exposed is unsafe, such as a giant horde of chaingunners. In these cases using the Plamsa Gun might get you shredded by a wave of enemy projectiles or hitscan shots. There is a great tactic you can use with the BFG that lets you deal a reasonable amount of damage while significantly reducing the amount of time you stand out in the open.

Hide behind a wall facing in the direction you want to deal damage in, and stand close to the edge. Then fire the BFG directly into the wall and as soon as the projectile leaves the gun, briefly strafe out into the open. If you are close to the wall it will immediately detonate, giving you about half a second to move away from cover and let the aftershock shots fire towards the enemies. Then you can immediately duck back behind cover and hopefully take no damage. This is also a great tactic for quick Deathmatch kills against other players.

If you can't get reasonably close to your target, the BFG is usually not worth it unless you have a very large and dense line of enemies to hit. The Plasma Gun is more efficient at range.
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ClasSixxS Jun 22 @ 7:25am 
Thanks. Novice Doom player here. Learning all sorts of stuff about this game.
RoyalRB May 26 @ 5:11am 
Thank you so much for this guide! I knew about the cone, but not that it doesn't matter what way you face. I also didn't know about the random damage and the hitscan shots. I was so confused why my bfg shots were so weak, but with the help of your guide, I am now able to fire more effective bfg shots. I appreciate all the effort you have put into this guide and I have read it all.:DukeExplosion:
No HL3 :( - RųŞɰ@ Sep 6, 2017 @ 8:40am 
Good Guide.

As I played Doom II a lot I was confused regarding so many damage outputs. It clears most of things. Doom II is one of the... In fact the most hardest game (in NIGHTMARE mode) I have ever played. Still didn't manage to finish it in (Nightmare mode). After this guide lets Starts again.
Gordon Jun 11, 2017 @ 12:30am 
spunk got the fan name right but the true name is billion fireball gun or BFG for short
Doctor Omn Nom May 23, 2017 @ 7:38pm 
I would disagree and would say that the basic shotgun is Doom's most famous weapon, but good guide overall.
Prism Sauce Jan 1, 2017 @ 6:24am 
Not sure how relevant my information is to this comment section considering I'm over a year late, but yes, BFG stands for "Big Fucking Gun".
ShMo Sep 18, 2016 @ 5:35pm 
On Nightmare! (or at least when you turn on respawning enemies) the BFG becomes far more helpful. In DOOM II, the level "Gotcha!" pits you against a Cyberdemon, a Mastermind, and (after flipping a switch) an Archvile who enters the large room. Took me multiple tries to have the Cyberdemon and Mastermind infight so I could finish off the Cyberdemon with a point-blank BFG shot, flip the switch, and get rid of the Archvile before the other two respawn and destroy me.
Mr.Spring Jul 27, 2016 @ 9:30am 
100% correct
AstroNut Nov 21, 2015 @ 2:32pm 
Yes, but what does BFG stand for? I think it stands for Big Fucking Gun
BobMcCow Sep 27, 2015 @ 2:47am 
All my assumptions about the BFG 9000 were wrong! Next you'll be telling me it doesn't stand for Big Fwoj of Green. :steamsalty: