there are a few issues:

The idea that he never visited the school before the day of the shooting; who says? How do they know? Even if he hadn't, he could scout via Google Earth etc. and floor plans, fire maps, etc. can be found on line so he can find entrances/exits, etc. Also he could measure fairly accurately the distances from various points to where the stage would be so it's not as big a deal as LJ thinks.

As for the bullet, they are actually talking about a bullet fragment so, depending how large it is, it's not unusual for them not to match the rifling from the gun. Pretty much all pattern-matching forensics is bullshit anyway, but it still plays with juries so this could be big for the defence.

If they can really make them understand reasonable doubt, there should be plenty of it they can point to. Could be hugely embarrassing for the feds.