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The single-bullet theory, often derided and discredited by referring to it as the magic-bullet theory,[1] was introduced by the Warren Commission in its investigation of the assassination of U.S. President John F. Kennedy to explain what happened to the bullet that struck Kennedy in the back and exited through his throat.[2] Given the lack of damage to the presidential limousine consistent with it having been struck by a high-velocity bullet, and the fact that Texas Governor John Connally was wounded and was seated on a jumper seat 1+1⁄2 feet (0.5 meters) in front of and slightly to the left of the president, the Commission concluded they were likely struck by the same bullet.
Generally credited to Warren Commission staffer Arlen Specter[3] (later a United States Senator from Pennsylvania), this theory posits that a single bullet, known as "Warren Commission Exhibit 399" (or "CE 399") caused all the wounds to the governor and the non-fatal wounds to the president, which totals up to seven entry/exit wounds in both men.[4]
The theory says that a three-centimeter-long (1.2") copper-jacketed lead-core bullet from a 6.5×52mm Mannlicher–Carcano rifle fired from the sixth floor of the Texas School Book Depository, passed through President Kennedy's neck into Governor Connally's chest, went through his wrist, and embedded itself in Connally's thigh. If so, this bullet traversed 15 layers of clothing, seven layers of skin, and approximately 15 inches of muscle tissue; it also struck a necktie knot, removed 4 inches of rib, and shattered a radius bone. The bullet was found on a gurney in the corridor at Parkland Memorial Hospital after the assassination. The Warren Commission found that this gurney was the one that had carried Governor Connally.[5] This bullet became a key exhibit for the Commission. Its copper jacket was completely intact. While the bullet's nose appeared normal, the tail was compressed laterally on one side.
https://en.wikipedia.org/wiki/Single-bullet_theory