I have a airburst round with set amount of fragments with a certain shape inside my bullet. After a certain distance has travelled, a charge explodes in the back of the round disintegrating the shell and sending the fragments out in a forward cone.

My problem : 

1. Why do they shoot out in a cone? Or put in another way, why does a shotgun shell's fragments form a cone spread?
2. Would it be possible to choose a charge size/shape so that the discharge angle can be controlled?
3. Should every fragment be modelled separately after the explosion or can they be modelled as whole?