There is or was a gun shop owner in Baltimore that was asked to repeat that shooting since he was a well qualified marksman. He did it with time to spare using a like rifle.
You are probably referring to Howard Donohue, who was one of eleven expert marksmen (Maryland state troopers, ballistics-lab employees, Vietnam vets, etc.) who were asked by CBS to participate in an elaborate reenactment of the Kennedy assassination (and “test” of the Warren Commission hypotheses) on a reconstructed set at the H.P. White Ballistics Laboratory in Maryland. Donohue and the eleven participants were given three chances to fire three shots at a moving target dummy in less than 5.7 seconds and score at least two hits. Donohue failed the first two times, and on the third time managed to score three hits in 5.2 seconds. He was the only one of the eleven to beat Oswald’s alleged time of 5.6 seconds with at least two hits.
But you probably don’t want to cite Donohue in support of a lone-assasin theory or a single-bullet theory. In addition to being a gun-shop owner, Donohue was a ballistics expert and serious student of the Kennedy assasination. Donohue believed, until the day he died, that Kennedy was hit by two very different bullets - in the neck with a 6.5 mm bullet consistent with Oswald’s Carcano and in the head with a thin-jacketed .223 cal round consistent with an AR-15.
Donohue’s evidence was written up in the book, Mortal Error by Bonar Menninger. Donohue’s thesis was that the head shot was fired accidentally by a trailing Secret Service agent amidst the chaos and confusion following the first two of Oswald’s shots (one of which hit Kennedy). While subsequent investigations have shown this to be high unlikely, it may that the .223 bullet was fired from a separate rear location.