our last mass shooting happened in 2001, when a mentally ill person (Friedrich Leibacher) assaulted the Kanton Zug (like on of your states) parlament, killing 14 people, wounding 15 more, before killing himself.
From what I remember he was feeling persecuted and wanted to "punish" the one he considered guilty of all his problems.
He never used military assault rifle (even if he was carrying a civil version of the SiG550, he did not shoot it), but a shotgun, handguns, and a couple of selfmade pipe bombs.
This could be avoided, because the police of this Kanton KNEW he had guns, KNEW he was mentally unstable and dangerous, and still did nothing. But the curent laws allowed them to take his guns away in a preventive action , but still did they nothing.
http://dic.academic.ru/dic.nsf/enwiki/4143950
Why making new laws when you dont apply the one that exist ???
3 other cases in the last 5 years did hurt us pro gun people.
-One Iraqi, freshly Swiss naturalised, used his issued SiG550 to spray and pray a restaurant in which there was someone he did not like.. no casualty, but a couple of wounded.
-another "new" Swiss (from Chile), the day he ended Recruit School (like your boot camp) and was coming home with all his gear (including his assault rifle) camped on a hill, and used his rifle to "snipe" a 16 years old girl that was awaiting for a bus.. I think there was no justification for this, and this guy was declared just crazy, and shoot randomly.
http://www.tsr.ch/tsr/index.html?siteSect=200001&sid=8469541
-A militia Leutnant used his issue SiGP226 to kill his wife (Corinne Rey-Bellet), and if I remember well her brother, for a "love affair" of some kind. This event was really hammered by our media, since Corinne Rey Bellet was a Alpine Ski champion, who won many races of the world cup in her career.
http://en.wikipedia.org/wiki/Corinne_Rey-Bellet
That's about it. In most of the cases, people were just "crazy" and never showed it before... so there was no way to prevent that from happening (exept not considering an Iraqi boy, which culture isnt the same as ours, even if he's got a swiss passport for a year..) .. And when it could be prevented to happend, the police was guilty of not doing what the law allowed and suggested them to do.