One of my favorites is Arsenal. Feels like a nice little source Valve game like counter strike, except its a gun game.
Arsenal or MM2 are good FPS games on roblox.
I suggest you to play Polyguns and CB:RO. First game is like Overwatch and second is like Counter Strike: Global Offensive. By the way, I can't find Polyguns on site so I think it had been deleted, it's sad 
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As title says, are there any good fps games inside Roblox other than PF?