The bullet pulled from the victim is described as being too fragmented to get striations. However, the bullet shown has the base of the bullet fully intact with only the nose (where there would have been no striations) mushroomed with clean, obvious, and easily visible striations showing on the base of the bullet.
The lab crew ostensibly sifts through many buckets of nuts looking for a shell casing; all they'd have to do is x-ray each bucket to detect the piece of metal.