Most military rifles of the 19th-to-20th-turn-of-the-century era, not just Carcanos, shoot a foot or so high at 100 yards with issue sights and military or equivalent loads. That is because the battle sights are zeroed for 300 yards or meters. Soldiers were instructed to aim low for shorter ranges. At 100 yards, you aim for the belly for a center-of-mass shot. It does take a higher front sight if you want to be dead-on at 100 yards, but nothing like 6", more like 3/4".
Regarding the Kennedy assassination -- virtually every "fact" in the popular books about it has long been shown to be pure bunk. Oswald and his rifle were very capable of making the shot, his position was almost ideal, and the "magic bullet" followed exactly the course one would expect -- IF you know the facts. For instance, the rear seat of the limo was mounted higher than the front, and Connelly was leaning and partially turned when he was hit. It all lines up perfectly.