The difficulty was not the shot. The difficulty was in the time frame.
Carlos Hathcock:
"Let me tell you what we did at Quantico. We reconstructed the whole thing: the angle, the range, the moving target, the time limit, the obstacles, everything. I don't know how many times we tried it, but we couldn't duplicate what the Warren Commission said Oswald did". (KILL ZONE, pp. 89-90).
“trying to duplicate the feat of 3 shots in 5.6 seconds on a moving target in a high to low angle. He said after several tries they simply could not duplicate it”
Three shots in 5.6 seconds are what was not duplicated.