I'm no JFK expert, but if I recall, there was not a whole lot of time between the shots. That was one of the arguments made by the two shooter camp was that Oswald was not a good enough shot to get all the rounds off in the time frame with a bolt action weapon. That was proved a weak argument after looking at his boot camp scores though. Anyway, there was far too little time for someone to think about their ammo choice, grab a different round, load, then fire it. If different rounds were used, they'd have had to come from different guns. That or have two different kinds rounds loaded in the magazine.
I'm sure I probably have some detail wrong there- go ahead, rip it apart Glen