I am sure a lot of bullets can’t be perfectly traced back to the gun that shot them.
So? This doesn’t really change anything.
Correct. It means absolutely nothing.
They don’t have a bullet, they have a fragment. Think of taking an egg, making breakfast, and then the FBI comes and picks up a quarter-inch eggshell they find on your street. They are going to get SOMEONE for littering.
All the neighbors point to you, because you were the only one cooking eggs that week. But, the FBI can’t prove it because they can’t match the fragment to your frying pan. (Poor analogy, but...)