I'm currently working on a music based space invaders game, the player has four types of ammo to choose from which they have to match up to the the right enemy. I want to emulate a similar behavior of the old shooters where only one bullet could exists on screen at a time thus avoiding the player spamming and increasing tension. Any ideas how to do this I keep trying to think of compare actions or something to work it out but I'm not exspirenced enough to quite get it.