Enemy shoots At A Variable

I’m having some problems with the attatched FLA.

Ive got three main movieclips, a blue circle, a red circle and yellow bullets. The user controls blue, and red is the enemy and shoots yellow bullets at you.

I’ve got the shooting down, it shoots towards the user etc. What I’m trying to figure out is how to get red (which is enemy AI) shoot by a variable. So basically it shoots one bullet every set interval, which can be decreased (So more bullets are shot) as the levels get harder.