I really like Planet Side.
Its pay to play, massively multiplayer and it is kind of "sci-fi" but most of the guns are rifles, shot guns, machine guns and RPG type weapons.
The game gives a very good infantry+ light armor feel. There are lots of cool vehicles and transports and as you build your character, you get more points to buy certifications on different weapons, armor and vehicles.
The game is played usually with squads of up to 10, but you can have a dozen or more squads from 3 Empires fighting for one base!
The landscapes are HUGE and its just a really meaty game if you got the time.
I play on the Johari server in the New Conglomerate (Libertarian) faction as Pendragon - usually 10pm-2am PST.