About autofire and targets:
The normal course is to target the 10x10 area, defense 10, ref DC 15 negates. 10 Bullets
With Burst, you can fire 5 Bullets only, and target a single enemy, taking -4 on the attack and adding 2d to the damage.
With Burst and a weapon with burst setting, it's as above, but with 3 bullets.
You can also use the normal attack (10) bullets to attack a single enemy, but then it's a normal attack and you waste 9 bullets (I'd also houserule it that you could use the burst setting of certain weapons to make a normal attack with 3 bullets, since it's no knack, but a setting of the weapon. You won't be able to use the burst as effeciently as the Burst feat users, but you don't have to waste all those bullets.)