If you want virtual warfare, this is currently the gold standard. I could go on and on, but, if you have Half-Life, you must get this mod and try it for yourself. Don't get discouraged if FA seems complex at first. Give it a little time. You'll love the weapons and customization features.
Weapons are ultra-real, both visually and functionally.
Many death variations, from 20 foot airborn trajectories to sudden uber-splatter. OpenGL card with 16 MB minimum. Get FA and you'll hear what I'm taliking about. Firearms has a larger assortment of real world weapons, When you get shot you bleed, and if you dont bandage you die. Not too much like rainbow six, but not arcade like quake.