Getting back to the original question, and looking into the pondering, gun laws do work. But only that far, they never work as a complete insurance against getting killed by a bullet.
Looking at mass murders is one thing. Looking coldly at pure statistics, the USA sticks out as the western country with the highest percentage of capital crime, - murder, rape, armed robbery. The average of murder is actually about the same in the rural parts (alltogether) in the USA, as in London.
The answer IMHO is possibly that the USA is just full of guns. The access is easy, conceilable weapons widely allowed, and even more high powered weapons which have all the potential of hosing down a whole classroom.
Putting some regulations into it will take a long time to work. If I recall it correctly the British tightened their rules some while ago, and offered people to turn in weapons of no use with no questions asked, maybe there was even a bonus in it.
There is perhaps a second answer. It's just the wild old culture of a "gunland"