I have to pause between 'A' and 'FC' in order to say it properly.
So I say 'An FC' because I don't have to pause in order for it to work in a sentence.
However, if you are speaking without terms. 'A fullcombo' works better than 'An fullcombo.' Perhaps it's because of the acronym of 'Fullcombo' that makes it easier to say aside from 'A FC'.
But then again, that depends on how you say 'A FC'....dear I could go on for hours with this.