I kind like agree with Plane Sailing. The fluff of the power must make sense, but I go another step and say that the fluff is up to the player capacity of make up a excuse 
If they go and say: "I'm using BB in these two drake swarms" and rolls, I would just say: "HOW!?". Then is up to then explain. Targeting the leader, using chalk, saying his magic dagger turn in hundreds for this attack. If it makes sense in the game world, and doesn't change the mechanics I let then try. I do this for everything bizarre, like droppoing prone the Gelatinous Cube.

If they go and say: "I'm using BB in these two drake swarms" and rolls, I would just say: "HOW!?". Then is up to then explain. Targeting the leader, using chalk, saying his magic dagger turn in hundreds for this attack. If it makes sense in the game world, and doesn't change the mechanics I let then try. I do this for everything bizarre, like droppoing prone the Gelatinous Cube.