The BFG-9000 deals damage in two different ways. The first is straightforward, the second much less so. The first way in which the BFG deals damage is with the projectile, which deals 100-800 damage in multiples of 100. The second is with the tracers, which fire 16 frames (or about .457 seconds) after the ball strikes something. One BFG shot will release 40 tracers in a 90 degree spread, and each tracer is individually affected by autoaim. The center of the spread will be your position at the time the tracers fire, but in the direction in which you fired the ball. That is, if you moved, this affects where the tracers shoot from, but just turning does not.

Each tracer that connects with an enemy rolls a number from 1-8 15 times to determine the damage dealt. Because of how Doom's rng table works, this will always result in a number from 49-87 in vanilla compatible engines. If the ball plus all 40 tracers hit something, this will deal an average of about 3,134 damage, (in ports with more sophisticated rng, such as GZDoom, this goes up to 3,150). Thus, with good rng and VERY good luck with the blockmap bug (which is a topic for a WHOLE other conversation) it is possible to kill a spider mastermind with only one bfg blast, and killing a cyberdemon with two shots is fairly consistent once you have the technique.

With a fundamental understanding of how the big, uh, freaking gun works, a few techniques become apparent. Firstly, to maximize damage, you need to be close to a target. By getting in a cyberdemon's face as the tracers fire, you can make sure that as many as possible connect (at least when Doom's collision decides to work, lul). I usually like to roughly line up my "fire" input with the "whoosh" sound from the first rocket, firing slightly sooner if I'm further away than I'd like or later if I'm a bit close; in any case, you need to close the distance before the tracers fire by either weaving between the rockets or dodging to one side. There's really no substitute for just practicing the technique a ton and developing a strong muscle memory for it, so I'll link Nine Inch Heels's 15cc.wad here:[dsdarchive.com]. The original challenge associated with this wad was to kill all 15 cybers in under 2 minutes, so good luck!

The tracers can also be abused by shooting a wall and quickly stepping into an open area with a lot of enemies, allowing you to deal a lot of damage while exposing yourself for only a short time. This one works great in deathmatch! Additionally, you can often initiate a fight by "pre-firing" bfg blasts. If timed well, you can blast enemies with a ton of tracers as soon as they spawn in or are revealed by a lowering wall. Both of these techniques work well against arch-vile hordes. Here's an example of these techniques being used in the final fight of Stardate20x6 MAP07.

Hope this guide serves you well, and let me know if you have any questions or corrections in the comments. Happy hunting!