The small round hole in the rear of JBC's assassination-day hole was enlarged twice to examine fibers for trace elements.
Even so, it is a small round hole, just large enough to accommodate a non-tumbling shot from a WC 6.5 slug.
The wound in JBC’s back was elongated and the bullet hole in JBC’s suit was elongated and you think the shirt that was sandwiched between them is somehow going to be different? Any doubt why the FBI showed little interest in the shirt?