How do you know that they matched? We have only the statements of the FBI....
You are absolutely right. I was left with only the word of the FBI that the bullet that they say they fired with the real C2766 rifle and on which the groves align perfectly with those on CE399 was actually fired by the real C2766 gun that was linked to Oswald.
But that is not fair and assumes that there was no massive conspiracy to mislead the Warren Commission and cover up the fact that CE399 was not only planted by conspirators but that the conspirators had also planted an identical but different MC with the exact same markings and same serial no. with the FBI.
The Warren Commission should have offered to have me, and anyone else who wanted to, to fire test bullets with the real C2766 and do the microscopic analysis of the rifling grooves.