You are going to love me and love me and love me for this (everybody the reputation button is on the upper right of this post which looks like a clipboard xD)
This is now a game that is by Ijji that is called Soldier front, or sfront for short. it normal has 2000 players + at any time of day (10k usually at night) and it's completely free!
the adress is:
First make an account,
Then say play live and you'll install it onto your computer (no viruses)
then to play it simply either play soldier front on your desktop (which brings you to sfront.ijji.com to hit the Play Live button) or go to sfront.ijji.com and press play live! you should go through a thing where you buy all your guns and here are the basic commands:
a: left s: backwards d: right w: forward mouse: to aim left click: shoot right click : zoom (If possible) space: jump and ctrl: crouch and the rolling thingy between the mouse: scroll through your weapons.
here are some screen shots:
have fun with guns!!