It's not a separate viewport. In most games, in first person view, they occlude the player model, lower the camera to about upperchest/neck height on the player model, and then display some guns on the screen irregardless of FOV. This is why when you change the FOV, the guns stay the same, since they are independent of the FOV. They're more dependent on the screen resolution itself. Moving these would require additional tweaks.
Just think of the player's hands and guns as being rendered like the HUD. It's just an overlay on top of the scene.
Saying it's an overlay would suggest it has it's own render view and is composited onto the normal environment view though, sort of like a skybox? I guess technically not a separate viewport.
The Unreal engine is actually the only major FPS engine I know of where the gun models aren't affected by changes to the FOV though. FOV changes in Quake, Doom 3, and Source engine games do affect how much of the the first person weapon models are drawn - it seems like their a part of the main world viewport, whereas in UE it seems they are not. Obviously there's some kind of difference between how these engines are positioning and drawing their first person models.
Anyway, main point is even with FOV changes, UE is still has widescreen flaws compared to other engines.