In HRC hunt tests and actual hunting a dog that knows how to “mark off the gun” 
    has an advantage. My dogs move through a transition of training drills in learning
    how the “swinging” gun aids in marking. The first phase in the process is acquired 
    from the OB drills teaching push/pull. The next step is using push/pull with two 
    bumpers in the yard. The dog retrieves after two tosses at about a 20° angle. The
    dog  is randomly pushed or pulled (physical influence) without moving their butt to
    teach the direction to look. Here are a few links to teaching “push/pull”.     
          note: You may have to scroll to find them.

                                                Pivoting: One Step to Tight Lines (link) 
                                                     Platform Tips - push/pull (link)
                                           
           note: A young dog needs to first learn about marking without the distractions of
                      a gun and/or a lot of action at the line (in my opinion). 

    In HRC and hunting, what the dog must look for may not be in that tight of  an angle  
    (20°). In those instances, remaining stationary is not always practical. Therefore, the
    next step (for me) is what I call the 180° bucket drill. This is where I introduce the gun.
    The rationale for the large angles is that huge motions of the gun are more clearly
    understood.  Actually, it is a cap gun with a PVC barrel extension. This produces a
    "noise" while allowing a free hand to "assist" the dog. This bucket drill consists of
    three identified piles of orange bumpers. I generally have the dog move with "here"
    and "heel" to line each pile before adding in the cap gun's effects. The pointing cap
    gun motion (aimed at various piles in random order) is the initial imprint of "moving
    with the gun". It isn't long before the dog "sees" the cap gun barrel as selecting the
    direction to look (almost like a cast).
                                                             Cap Gun Training (Link) 

    The final part of the transition requires simple setups with two wingers. In these
    training sessions, the dog is sat so as to be facing a direction that splits the angle
    between the marks. The dog is then pushed or pulled with the gun (cap gun or
    shotgun) to the marks (either as singles or a double). Repetition is critical at this
    point to give depth to the new expectation. In the last part of this skill development,
    there is an effort to keep the dog stationary by positioning him so that he can see
    both marks by just moving his eyes and/or head.

          note: the cap gun is used when there is a need for more focus on the dog at the
                     line (extra hand = more control) 
          note: the winger remote is Velcro'd to the pump action of the gun (finger tip
                     control)

    To repeat, prior gun conditioning is important before starting this drill work. In the
    "marking off the gun" training, "noise" progresses from cap gun, to popper gun with
    primer loads and finishes with the louder, full popper loads. On a side note....duck
    calling is mixed in as needed. The entire process is done very gradually within the
    context of a regular retriever training program and often spread out over several
    months.

    After a steady progression of training for "marking off the gun", regular maintenance
    (repetition in training) plus real hunting experiences will have a dog very adept at
    knowing where to "look".  Get in the swing.