Based on their examination of the bolt-action, clip-fed, Mannlicher-Carcano rifle, firearms experts for the Warren Commission established that at least 2.3 seconds per shot would be required for Oswald to execute the assassination. The shutter speed of Abraham Zapruder's Bell and Howell movie camera operated at about 18.3 frames per second. After numbering each individual frame, tracing the movement of the vehicle, and taking into account other factors, the Commission hypothesized that since the view from the snipers nest would have been obscured by the foliage of an oak tree between frames Z-167 and Z-210, the earliest the president could have been shot was Z-210. Zapruder's view of the motorcade was blocked by a road sign between frames Z-207 and Z-225, but when Kennedy appears from behind the sign he is beginning to react to the throat wound. The commission therefore reasoned that the President was shot in the neck between frames Z-210 and Z-225. Based on Connally's reactions in the Zapruder film, he was apparently struck between Z-236 and Z-238. These observations, however, were puzzling. If Kennedy was struck in the back at Z-225 and Connally at Z-238, that would entail a 13-frame time span or 0.71 seconds. But that would have been impossible since Oswald would have needed at least 2.3 seconds to fire two shots. Even if Kennedy was shot as early as Z-210, that would still be only a 28 frame time span, or 1.53 seconds. Therefore, Oswald apparently could not have shot both Kennedy and Connally. Logically this left the Warren Commission four options: abandon the lone gunman theory, assume that Oswald somehow managed to hit the President while shooting through the tree, lower the 2.3 second estimate of the time required to operate the rifle mechanism, or assume that a single bullet struck both Kennedy and Connally. The Commission took the fourth option, known as the "single bullet hypothesis," and concluded that three shots were fired within a 4.8 to 7 second time lapse, and that at least one of the three probably missed the target, although they could not determine with certainty which of the three missed....

Assassination of John F. Kennedy Encyclopedia

Since the Warren Commission elected not to extend the time frame in this way, they concluded that the entire assassination took place between frames Z-210 and Z-313, or a time span of 5.62 seconds, just enough time to get off three shots. Any competent gunman would have had the rifle already cocked for the first shot, in which case the bolt would have been operated only twice and the absolute minimum time expended operating the bolt would have been 4.6 seconds. But if four shots were fired, the rifle would have been cocked three times requiring 6.9 seconds. So, if four shots were fired there must have been more than one gunman at Dealey Plaza. Of course, if the Commission had accepted the possibility that the first shot was fired through the tree at Z-166, then their estimate of 5.62 seconds could be extended by 2.4 seconds to about 8 seconds, more than enough time for a lone gunman to get off even four shots.