If the likes of Call of Duty or Battlefield offer far too casual an experience for what you're currently seeking, then the ongoing Humble Arma Bundle should provide the perfect answer. Bringing together the mostly military tactical shooter games of the Arma series, you can expect at least dozens of hours of simulation-like action from the bundle.
At RM4.45 ($1), you get Arma: Cold War Assault, Arma Gold Edition and Arma Tactics. Cold War Assault is a re-release of 2001's Operation Flashpoint, while the Gold Edition contains the follow-up Armed Assault and its expansion, Queen's Gambit. Tactics is the most recent release out of this bunch, and stands out for being a turn-based tactics game instead of a shooter and having a "mostly negative" rating on Steam.
If you decide to surpass the average of RM57.54 ($12.93), the entire Arma 2 collection will be yours. That means you'll be getting Arma 2, the standalone Arma 2: Operation Arrowhead and the DLCs British Armed Forces, Private Military Company and Army of the Czech Republic. Pay at least RM66.75 ($15) and Arma 3 is yours, along with the kart-racing Arma Karts. The latter started off as an April Fool's Joke, but it became a DLC for Arma 3 later on.
While you're at it, you can also check out Project Argo. While not part of the bundle, it's advertised on the Humble Arma Bundle page and is free. The prototype is an "experimental total conversion of Arma 3" where two bands of mercenaries engage in 5v5 battles over objectives, and is labelled as a Bohemia Incubator game.
At time of writing, there's 13 and a half days till the bundle ends. That's plenty of time to decide on whether you want to dive in, but if you get it now and end up getting sucked into the world of Arma, I suspect that you'll be playing long after the sale ends.