Big Bang is the best, bar none, set of rules for simulation of real combat firearms. Useful for adding flavor to almost any rules (I love it for Fudge!) you can have your gun rules as crunchy or as fudgey as you like. The firefight is hot and heavy? Go with the quick way and you'll still have a shoot-out more realistic than almost anywhere in RPG land.
All Big Bang guides are highly recommended!
[5 of 5 Stars!]