well for f2p FPS games i'd say you try wolfteam.
it can be laggy sometimes (rarely for me)
you can transform into a wolf if you want to
aim very accurate and its a 1 hit kill by a headshot
(when i used to have the AK47 i could do a headshot from over half the map lolz, and of course i got votekicked alot because they thought i was hacking, but hey thats what you get if higher lvls cant accept that they got owned by a low ranked person)
also there is one FPS to give you the full experience of being in the army
America's army
havent tried it in a few years myself but maybe you can find it entertaining