Low poly sight on Security pickup truck machine gun
The issue rarely happens when the a game does not load proper LOD (often due to slow hardware), so if this is the only issue you are seeing, it'll be fixed in next match you play.
If you see it persistently, along with other LOD issues such as the entire vehicle being low-poly, hands are open in 3rd person instead of closed while holding a weapon, etc. then you used some random Engine.ini config from the internet forcing lower quality LODs to be displayed, in which case remove it and adjust your in-game video settings as per your performance target.