I have an instant-fire weapon which sends a trace for each shot. I have added a bullet tracer actor which acts as a bullet tracer effect (has a particle effect attached to it). The bullet tracer actor is created at the same time the line trace is made. This results in the trace hitting this bullet tracer actor.
If you want to ignore a specific instance of an actor you just have to add it in the CollisionParams object. For example if you want to ignore the Actor Player when doing a LineTrace: