[Top Dog]

you might or might not be allowed to tap the trigger once on a burst fire weapon and swing the weapon around very fast while the weapon automatically spit out three bullets so each bullet land on up to skill / 2 targets (which might or might not be more than the number of bullets you fire) that might or might not have to be within short or medium range (only a limit on the long burst version). there are no rules how to resolve it and common sense tell us that it is not really viable. if this (or parts of it) is supposed to be ok then it will have to be described in a future errata (or house ruled)

While I wouldn't mind you houseruling this out since the rules are somewhat unclear (and it's not really common sense), it is specifically allowed in the rules:
"Burst Fire weapons can take advantage of the Multiple Attacks Free Action to fire at multiple targets with the same burst."
(This is under the 3-bullet burst rules).

[Agonar]

What I ment was, if you attack, say, 6 targets with a long burst, 1 bullet each, do they take a -5 dodge penalty each for receiving a long burst? The rules say you take the -5 if the attacker did a long burst. It says that's because of the amount of bullets flying at you (so RAI is that it's dependant on that), but it doesn't actually make it dependant on that in the actual rules, and Multiple Attacks is silent on it as well. So RAW all 6 targets would receive the -5. Which is silly.

Page 180, the red box titled Not Enough Bullets answers this. 

For example, Wombat attempts to fire Full Auto (Complex
Action) but his Ares Alpha only has 7 rounds left. 10 bullets
needed minus 7 bullets left is a 3 bullet shortage and a 3 point
reduction in the –9 defense modifier, making it a –6 on defense.
Full Deck fires a Long Burst that empties the last 5 rounds
from his Uzi V. Since he is 1 round short he only imposes a –4
defense penalty to his opponent.

The defense penalty is reduced by 1 for every bullet short of the "required" amount going at a target.  a 6 round burst imposes a -5 Defense penalty.  If you only have 4 bullets left, you are 2 bullets short, so the defense penalty is -3.  If you are splitting it so that 1 bullet flies at each of 6 targets, then each target is 5 bullets short of the "required" amount, so the -5 becomes -0 for each of them.  Another way to read this would be that the Defense Penalty suffered by a target is equal to one less than the total number of bullets being fired at them. 

At this point, reducing your dice pool among so many targets, while they all get to roll full defense pools, just makes it very likely that you will; Miss, Glitch, or Critically Glitch.

[Top Dog]

Page 180, the red box titled Not Enough Bullets answers this. 

For example, Wombat attempts to fire Full Auto (Complex
Action) but his Ares Alpha only has 7 rounds left. 10 bullets
needed minus 7 bullets left is a 3 bullet shortage and a 3 point
reduction in the –9 defense modifier, making it a –6 on defense.
Full Deck fires a Long Burst that empties the last 5 rounds
from his Uzi V. Since he is 1 round short he only imposes a –4
defense penalty to his opponent.

The defense penalty is reduced by 1 for every bullet short of the "required" amount going at a target.  a 6 round burst imposes a -5 Defense penalty.  If you only have 4 bullets left, you are 2 bullets short, so the defense penalty is -3.  If you are splitting it so that 1 bullet flies at each of 6 targets, then each target is 5 bullets short of the "required" amount, so the -5 becomes -0 for each of them.  Another way to read this would be that the Defense Penalty suffered by a target is equal to one less than the total number of bullets being fired at them. 

Except that's not what it actually says. It says if the burst is short a couple of bullets, the penalty is reduced. It doesn't deal with bullets per target anywhere, including not here.

Extrapolating that rule to this situation is the fairly logical thing to do, but that's not what the actual rules say - as the others have said, the rules don't really explain attacking multiple targets all that well.

[Xenon]

if you take a long burst you tap the trigger twice and fire two normal 3 bullet bursts (which you can aim at two different targets).

if you only have 5 bullets then the second burst against the second target will only fire 2 bullets instead of 3
(or the whole long burst against one target will only fire 5 bullets instead of 6).