"Yes, but Specialist Miller had very little experience firing Carcano rifles, probably a lot less than Oswald had.'
Miller was a Master Marksman who was actively competing and training.
Yes, but not with the Carcano rifle. The first time he ever used the iron sights of a Carcano rifle, was when he fired the three shots, all of which went high, particularly the third shot. Exactly as to be expected of a rifleman who can line up the iron sights with the target, but does not know that the Carcano will miss high by 5.7 inches at 88 yards.
Being an expert rifleman, as Specialist Miller was, does not mean that he can shoot accurately with all rifles, including the ones he has no experience with. With any expert rifleman, he has to be allowed to become acquainted with his weapon first, before he can demonstrate proficiency with it.
Expecting Specialist Miller to do well with the Carcano, or any rifle he is unfamiliar with, is like expecting the winner of the Indianapolis 500, an expert in driving Indy style racecars, to place in the top three that year’s Monte Carlo race, driving a Formula 1 racecar, of which he is totally unfamiliar with and has never driven until the day of the race when he dashes up and jumps into his Formula 1 racecar and starts it up, assuming he can figure out how to do so.