Insurgency is a tactical shooter and requires skill and teamwork to master. This game has a focus on realism and provides a very limited hud, usually the only thing visible is the objective. Dying is incredibly easy so careful gameplay is a must but don't play too careful otherwise you will lose the objective.
If you liked Hardcore mode in Call of Duty you will love Insurgency.
The game is using the opensource engine and runs on Windows, Mac and Linux. The graphics are really good for a source game and the scopes you use to zoom in look very realistic.
Its focus on realism is really well done, lightweight soldiers are useally down in 2-3 shots but you won't know for sure because there is no killfeed. You could check the scoreboard but it won't display your kill count until you are respawning again. You have to confirm the kill yourself.
I am not affiliated with the company but I can highly recommend the game if you like hardcore shooters.
You can purchase it here for just 1.49:
http://store.steampowered.com/app/222880/Insurgency/