Thanks!
Regarding bullet types- different types deal different amount of damage. Damage, as well as other bullet properties are linked to number of bullets you get- your get more if the value is closer to average, so that you don't get stuck with crappy bullets, or just rip through a boss, because you got good ones. I guess the differences in damage output aren't too dramatic (the best ones have little more than twice the damage of the weakest), so it may be hard to notice.