Seen some interesting Youtube videos of America's Army proving Grounds (AAPG). System requirements aren't too bad and it's still UE3 I believe.

Red Orchestra 2 was a good game, but didn't run well in Windows Vista. Would likely require a solid Windows 7 PC with good specs to run well. Other people get auto kicked for latency/lagg/"speed hacks". No there is no such thing as speed hacking, it's just a weird name for lagging.  Seems pretty realistic with different ways to ADS the sights and class based system.

Unreal Tournament 4 (or Unreal Tournament 2015) runs on Unreal Engine 4. It's in beta or alpha stage.

There are plenty of single player ones; BIA series (MP shutdown), Batman Arkham Series ect.