My first thought when they id'd the .223 from ballistics was "bolt varmint gun".
The only other evidence they have - aside from projectile fragments- is the spent case, which may have extractor/ejector marks that skew it towards a semiauto, or they may simply be rationalizing that nobody would eject a spent case from a bolt gun if they only intended to fire one shot. Either way, the evidence for semi auto appears weak at best.
If the case is bogus, we're back to anything goes.