Curved trajectories aren't actually possible in the game without some serious server side hacks. When a player fires a shot, a projectile is spawned somewhere near the gun's barrel end coordinates, with a vector that determines the trajectory and a probably numerical id that is used to identify the projectile type, speed, etc.. After that, the client (player's device) doesn't send any more updates about that projectile, every other calculation related to that projectile is processed by the server. The client will get an updated trajectory for that projectile though, so in theory it's possible that even you can sometimes see your bullets following a curved trajectory.
Also, a large field of view can make trajectories look curved, while the gun's barrel axis not being perfectly aligned with the crosshair and the bullet spawn coordinates can make the trajectory appear as a broken line.
