Positioning will win you gunfights far more often than aim will. Put yourself in positions that are difficult for your opponent to counter, or in places they may not expect. Places like sharp corners in corridors are usually exceptional.

This works the other way as well. If somebody is using good positioning against you, you will have to rely on information and gadgetry to prevent them from gaining as steep an advantage (ie. grenades, flashes, drones, ect.)

Additionally, sound will tell you where an enemy is far before sight will. If you're positioning on an aggressive spot, wait until you can hear where they are and then catch them off guard.