On one of the history specials last week, they stated that 3 spent cartridges were found on the floor - one left in the rifle ...
Interesting. The Carcano feeds from a 6 round en-block clip. You tend to load them a clip at a time, like a Garand. Italian milsurp ammo comes in 18 round packs, pre loaded into the clips.
We know Oswald fired one round at General Eddie Walker in April. (In that instance the 6.5mm bullet deflected when it passed through a window pane from 30 yards away.) I suppose he may have never reloaded but that would still leave one round unaccounted for.