1. They might be very hard to find, but the sabot pieces would be left behind at the scenes of the shootings somewhere just ahead of the shooter's position. I think that the investigators that are getting cadets to police the area would have come up with a few sabots by now if your theory is true -- assuming they're smart enough to know to look for them.
2. Small rifle bullets don't survive bullet traps well, even the water tank kind. If they do survive, they might not be very reloadable. If they are in enough shape to reload, they wouldn't be very accurate. Not accurate enough to hit someone in the head at the ranges this sniper is probably firing from.
Clever of you, though.
Really and truly, I was hoping someone would pursue the sabot thing (even in jest, as you did) to that point. There's at least one big practical objection, which I trust you and other knowledgable FReepers will point out.
Small hint, which won't need: why would someone use a sabot for criminal purposes?
(Mine will be better.)
I was saving that for my novel, along with a few other tricks--