Clearly it wasn't.
This assumes that Oswald brought the rifle in dissembled, which nobody really can say. Even so, he could reassemble the rifle quickly, and practiced doing so, and would no have needed the scope as the range was short.
The two key members of the Secret Service detail swear all three rounds hit the occupants of the car: Sam Kinney, who was the driver of the following vehicle, and Emery Roberts. The Warren Commission never asked them to testify and that seems to be a shame because if it is true then there is nothing left to talk.
This is from the 6th Floor Museum, at the 33:30 mark they discuss the shooting:
https://www.youtube.com/watch?v=EmCEx-f0dfI
This video is invaluable. Former Secret Service agents Gerald Blaine and Clint Hill provide sober and practical explanations for many of the so-called mysteries of that day.