Makeuseof.com published a post with the Top 5 free online shooter games. So you can have a check on their site and download them if you like. The games are

5. War Rock (Download size 550 MB)

4. Nexuiz (Download size 380 MB)

3. Warsow (Download size 180 MB)

2. Gunz The Duel (Download size 440 MB)

1. F.E.A.R. COMBAT (Download size 1800 MB)

Also you can check the games

• America’s Army
• Return To Castle Wolfenstein: Enemy Territory
• Mono

Check the post here