Even though the game is nice enough to tell you the odds every time you squeeze the trigger in XCOM, there's still an underlying feeling of "how the hell does this work", or sometimes "WHY THE HELL DID THAT JUST MISS".
The answer lies in numbers, and those numbers were last week very helpfully explained by Neil Brown, who in a couple of blog posts breaks down not only the facts behind shooting percentages, but also follows it up with an interesting piece on "save spamming", and how the game's "Iron Man" mode is about the only way, however imperfect it is, to stop people cheating a probability-based system.
If the detailed explanations and images are bit much, just stick to the graphs and know that "rapid fire" is a move that's statistically a lot better off than the game makes it sound.
Randomness vs Canniness, or Programmers vsSavescummers [The Sinepost]
Probability in Games: XCOM[The Sinepost]