I don't really see a problem, but the pulse phaser would definitely be visible on the outside of the hull.  A phaser beam is a stream of charged nadion particles (I think that's how you spell it.  I don't have my copy of Star Trek Science Logs here with me), and a pulse phaser constricts the release of those particles until they've built up to a certain level and then releases them in short bursts.  If you were to make a pulse phaser turret, I would think that you would have to put a large portion of the aparatus on the turret.