Unfortunately there no way to spawn objects by name at this time.
Easy solution to your problem would be use of instance variables for bullet sprite.
example in pseudo code:
bullet_type = 1 -> bullet.speed = 100
-> bullet.power = 20
bullet_type = 2 -> bullet.speed = 200
-> bullet.power = 50
then after spawning you need to set variable to proper value
+Turret spawn Bullet
+set bullet.bullet_type = 1