Which unpopular multiplayer FPS are you playing right now?

I really like Verdun, but it is much more slow paced

ballistic overkill is something my friend and i like to play when we are getting tired of the mainstream shooters.

you could try APB: Reloaded. Although it is 3rd person

Free to Play, I clocked over 900 hours before I had enough

Considering that Call of Duty 4 is 11 years old, we can say it is "unpopular" and not mainstream so I'll go with CoD4 :) Especially mods.

Darkest hour europe. Its a mod for the original red orchestra and a great game if you like something very hardcore/realistic. Usually enough players on most nights to fill a server.

Killing Floor 2 or Left 4 Dead 2. Those are PvE, I don't know if it's good, or you're looking for PvP.

http://store.steampowered.com/app/434570/Blood_and_Bacon/
http://store.steampowered.com/app/42670/Singularity/
http://store.steampowered.com/app/217140/Rise_of_the_Triad/
http://store.steampowered.com/app/233130/Shadow_Warrior/

I don't know of any good arena shooters, so can't recommend those.

I can recommend Insurgency which is a shooter that aims for a more realistic experience than generic shooters (no crosshairs or ammo counter, 1-2 bullets is enough for a kill with most weapons).

I tried Black Squad and it's a very generic F2P shooter. I wouldn't say it's a bad game though, I actually did enjoy playing it once I got used to it and the F2P model is surprisingly fair as you gain in-game money relatively fast and you can buy weapons permanently. I stopped playing it due to meh community though.

I was excited about the upcoming F2P shooter called Ironsight which seems to be heavily inspired by CoD games, but unfortunately since Aeria Games will publish it; the game will probably be pay to win. I'll definitely give it a try as soon as it comes out though.

Borderlands 1, 2 or Pre-Sequel?

anything from today's Humble Bundle

I guess you could consider L4D2 all these things you said nowadays. Really great game.

Just get DUSK - will be in EA soon (with Duskworld MP).

Singleplayer campaign already there to play.

http://store.steampowered.com/app/519860/DUSK/

Well, i like A.V.A , great old game ( still a game xD) but now has lot of p2w
I played it like years, last time... 1 month, not so far i guess

imagine csgo but if you pay for accesories you do extra damage

America's Army: Proving Grounds Something in between CS and Insurgency. Previous games from this series were much slower and realistic but yet gameplay still remains tactical. Fully F2P without any microtransactions.

Fortnite. Save the world. Not Battle Royale.

Combat Arms Reloaded

bad company 2. Still good

not 100% fps but it's good game

I am pretty sure Quake Champions is very much in the casual category, if we put CS:GO and Overwatch there as well. It is another thing that despite its F2P/P2W status, currently as I write this, SteamSpy shows a whopping zero people playing it. Then again, I do not expect anything better from a multiplayer game touched by Bethesda.

Rising Storm 2? Very underrated game, great squad fun, currently for a good price at Humble Bundle

Crysis 3 on ps3

Insurgency.600 hours.

You can get killing floor 2 and rising storm 2 pretty cheap in the humble bundle. Especially killing floor is a fun casual chill shooter.

Fortnite