I put more weight into "sworn testimony" vs "interviews". Officer Hargis gave WC Testimony that he ran to that "wall", "brick wall" so as to see atop the Triple Underpass. This "wall" being the wall running down from the Zapruder Perch. Hargis initially thought that a shot(s) had been fired from somewhere in Front of the JFK Limo. Fired from Up on the Triple Underpass. This was due to the physical reaction of JFK and the blood/brain matter being thrown back into Hargis. Over time, Hargis modified his impressions, but his WC Sworn Testimony stands. He ran to that "wall" to get a look at the Top of the Triple Underpass.
The blood and brain matter wasn't thrown backward into Hargis. It was dispersed forward and upward and Hargis simply drove through it. It's not a difficult concept to grasp.
The country road I live on used to have a bad pothole at one end. When it rained, the pothole would fill with water and I would usually forget it was there. When I drove through it, my windshield would get soaked. It was not because the water was being thrown backward. It was because the water splashed forward and upward just like the blood and brain from JFK's head. My car was moving faster than the water getting splashed so my windshield would go right through it. Same principle.