The Ammo

Cells

50 per battery

Also used by the hyper-blaster

Ammo Usage

50 cells per shot

The Damage

BFG10k	100/200 impact	500 detonation
		Laser Beam Range: 256 Game Units
		Detonation Effect Radius: 1000 Game Units
		Blast Radius Range: 200 Game Units

Firing speed

About one discharge (50 cells) every two seconds.

The BFG is the most complex weapon in Quake2. It has four different types of effects that all happen within a few moments of each other every time it is fired. I'll quickly describe those effects here, and delve into the weapon in greater detail below.

Impact

The most obvious effect is the impact effect. If the green ball hits a target, the target will most likely perish. This damage effect also carries with it a radius blast that is twice as damaging to the shooter as it is to the target.

Lasers

The next most obvious effect are the laser beams that burn out from the green ball as it flies through the air. While the green BFG ball flies towards it's target, it will actively seek nearby enemies and cook them with a green laser beam that is drawn from the ball to the target. That beam does 50 points of damage per second in 5 point increments.
The beams will only burn targets within 256 game units from the green ball. 256 units is about 20-25 feet.

Radius Effect

A large radius damage effect occurs very close to the detonation point of the green ball. This is there to keep you from firing the weapon directly into a wall or floor.
In the source code it dictates that the shooter of the BFG will take twice as much damage from the blast radius effect as any other target would. It seems that the designers wanted to punish a BFG shooter who fires the weapon to impact at close range. It's a good design decision as the greatest damage that can result from the BFG 10k is from the detonation effect.

Detonation Blast Effect

This is the serious damage from the weapon. It's somewhat difficult to understand, but later I'll provide some examples of how to test the weapon's effect yourself to fully understand its inner workings. The basic concept is triangulation. In order for a target to receive the detonation blast effect (up to 1000 points of damage) a line of sight must be able to be drawn from the target to the BFG detonation point and from the target to the player that fired the BFG at the moment of detonation. If any solid level geometry blocks such a line of sight from either object, the target will take no detonation blast damage. It does not matter what direction the player or the target is facing, and the shooter does not need to be able to see the detonation point. In fact, the shooter doesn't even have to be actually looking at the target. As long as a line of sight can be drawn, it'll cause the damage. Furthermore, relative position doesn't matter either. An opponent who stands behind me as I fire the BFG away from both of us towards a distant wall will be sufficiently cooked from the detonation blast if neither player moves before detonation.

In the above example assume the shooter is firing the BFG to the right while the target is behind him all along. When the shot is first fired, the target will likely get hit with some laser beam damage. As the ball flies towards the end of the corridor, the beam will be out of range and stop causing damage. When the BFG detonates, as long as no obstructions block the shooter's or the POD's line of sight to the target, that target will get cooked with the detonation effect and probably die.

Defense

For each of the different effects, there's a different strategy that helps to avoid damage. First, you should attempt to avoid getting hit with the actual green BFG ball. Then, you should determine where the green ball is going to detonate and make sure you're not in direct line of sight to that point when it explodes. Since your opponent will most likely be attempting to keep you within his/her own line of sight, your best bet is to worry about the ball itself. Once the ball is fired, it's path is predictable. Lastly, if at all possible, you should try to keep enough distance from the ball as it flies to avoid getting cooked by it's lasers. If you know you're going to take some laser damage you're better off charging in the opposite direction the ball is flying in as opposed to trying to out run it in the same direction it's travelling. Once you completely understand how the BFG works it's much easier to avoid than you expect. BFG wars with an opponent who also understands the weapon is one of the most exciting and enjoyable parts of Quake2 deathmatch.

Attack

Instead of firing the BFG towards your opponent, do a quick 180 degree mouse flick and and blast it into the floor behind where you were originally facing. If you continue to use your movement keys to drive you in the direction you were originally heading, you probably won't take any radius damage from floor impact and the transaction will occur so quickly that your target won't have time to get clear of line of sights before the detonation. It gets 'em every time.
Another trick is to fire the weapon directly into the ceiling of an open room that contains a target or two. The result, of course, is that any target with a line of sight to you (as the shooter) will most likely also be exposed to the BFG impact point on the ceiling and thus be cooked.
The main idea remains that you should detonate it as quickly as possible after aquiring a target.

Extra Info

A dead BFG shooter will kill any legal targets as long as line of sight isn't broken.
It is possible to fire the BFG, die and remorph within sight of the BFG detonation point and your old dead body. In that case you will have killed yourself as your new body will get pasted by the detonation blast effect, as a line of sight line can be drawn between from the player and the POD to the target - you're toast.

BFG Jumping

It is possible to use the BFG's blast radius to perform a 'BFG jump'. Like rocket jumping, the technique involves using the imparted velocity of the weapon's explosion combined with a jump to boost yourself higher into the air than possible with an regular jump. The technique is best performed by firing the BFG directly into the ground below your feet an instant after the jump button is pressed. Using a BFG jump will give you a slightly higher jump than with a rocket jump, but then it will take loads more health off you, so don't bother unless you have to (or you know there is some health up there).