I'm currently making a Playback System for an FPS I am building and can't figure out how I would decrease the speed of the playback to, for instance, show a bullet moving through the air towards a target, any suggestions?
It completely depends on your implementation of your playback system, but assuming you're using CFrame interpolation of some kind with delta time, it's possible to slow down the playback speed by modifying the total time to display it.
That is, if it takes 15.132
seconds to display the playback at regular speed, and you want to play it at half speed, simply double this time. That is, divide it by the fractional speed you want to reduce it by:
15.132/(1/2) == 15.132*2
If you want to play portions at regular speed and portions at reduced speed, or dynamically reduce the playback speed on the fly, that's a much more complex issue, and I won't be able to help you in the scope of this question.