Most Releastic Military Game Out
In B.C.T you do Obstacle Courses and Weapons training, you can even Jam your gun on this game and have to clear it. Once you get past B.C.T you can go farther and go Airborne and train for that, you can try for Special Forces, and more. Once you get a certain High rank you can be in Special Forces, and Customize an M4 Carbine to you Liking.
This game is the Best "In My Opinion" Military Shooter. What do you have to lose go try it, its free and you can Download it from gamespot.