I said in this post that no high-profile shootings of this type had been ended by members of the public exercising their Second Amendment rights.
In fact, there have been some cases. The shootings at the Appalachia Law School was one such, although the fact that the students who tackled the shooter were also armed was not widely reported, as this website discusses. The allegation of deliberate suppression of the detail that the subduing students were armed is analysized here. Wikipedia, as ever, has more.
Luke Woodham was also ’subdued’ by a non-law enforcement officer (the deputy principle of the Mississippi High School in which his attack took place) wielding a weapon.
AJJ’s post on the subject of the Virginia Tech shootings is well worth a read.