Turns out that bullet lead matching isn't really a valid way of determining whether or not ammunition found on a suspect was used to shoot a victim. The idea is simple; if a suspect is in possession of the same type of ammunition used in a slaying or shooting (caliber and bullet type, possibly seating dept, ...) A forensic lab could analyze the chemical make-up of the bullet, and see if two of them came from the same factory batch.
I don't really get what the commotion is about, the technique can show that a certain bullet came from a certain batch of lead. A defense attorney can reduce the legal value of that by pointing out exactly how many bullets there were in that batch. Bullets are made by tens of thousands (with indiscernible lead composition, actual batches are larger, but *do* differ along the way), chances are that all of the cartridges of a type sold in one gun store are from the same batch, so it doesn't really prove anything that isn't already established by finding the suspect in possession of said cartridges.
Now, it would appear that the method used wasn't of acceptable scientific quality. Frankly, I don't see what you're going to prove beyond that the suspect was in possession of the same type and brand of ammunition used. But hey, my colleagues just run the tests they're told to do by the DA/defense, they don't have to consider juridical relevancy.
If you're interested in the full story:
Fox: Hundreds of Inmates Wrongly Convicted By Faulty Forensic Tool
Washington Post: FBI's Forensic Test Full of Holes